Refactor ChainQuery to be more flexible

[evanlinjin]

Description

This PR introduces median-time-past (MTP) on canonical views. MTP needs block timestamps, and the existing task (bdk_core::ChainQuery) interface cannot expose them.

This PR changes the task (bdk_core::ChainQuery) interface to request block data by height, with the response now a generic B. A driver such as LocalChain can return Headers (which carry timestamps), and it resolves every requested height rather than collapsing the request to a single answer (as MTP needs all the blocks in a window, not just one). Previously the request was Vec<BlockId> and the response a single ChainResponse = Option<BlockId>: only the first in-chain block among the candidates.

The new response shape could have been done by keeping next_query() -> Option<…>, but the multi-stage canonicalization is much cleaner when the driving call returns a richer TaskProgress enum. This PR reshapes the protocol to a poll-based ChainTask whose TaskProgress expresses states next_query couldn't:

• TaskProgress::Blocked signals the task has issued every request it can but still needs outstanding data before it can finish. This is the hook a future streaming (non-blocking) driver would use to know when to wait.
• TaskProgress::Advanced lets a task write poll as one unit of work per call instead of wrapping its whole implementation in an internal loop. It also lets a driver observe progress in finer-grained increments.

Canonical views can then compute MTP per transaction (CanonicalTx::mtp) and for the tip (Canonical::tip_mtp), opted into via with_mtp() so callers who don't need it pay nothing.

Notes to the reviewers

• Naming. In the current master, LocalChain::canonicalize is the generic task runner and LocalChain::canonical_view is the high-level helper. This naming is confusing. In this PR, the runner becomes run_task and the helper becomes canonicalize.
• Relationship to CheckPoint::median_time_past. This does not replace that method. CheckPoint::median_time_past computes MTP for a height directly from the in-memory LocalChain checkpoint you already hold. Canonical-view computes MTP by sourcing its blocks through ChainTask / BlockQueries so it is driver-agnostic (works for any chain source, not just LocalChain<Header>) and attaches the result to each CanonicalTx and the tip, rather than making the caller walk checkpoints per height.
• TaskProgress::Blocked has no in-tree consumer yet — this is intentional. It exists for a future streaming driver (e.g. a Bitcoin Core RPC driver) that issues fetches without blocking: Query says "launch I/O", Blocked says "block until some lands". The only current driver (run_task) is synchronous and treats Blocked as unreachable!(). A comment in CanonicalTask documents the intended "anchored-leaves" batching optimization for the future driver.
• The shared anchor-checking logic used by both canonicalization phases is extracted into BlockQueries::resolve_candidates (returning BlockCandidateResolution), decoupled from Anchor by taking the candidates as (payload, BlockId) pairs.

Changelog notice

bdk_core

• Changed: the ChainQuery trait is reshaped into a poll-based ChainTask trait with a TaskProgress enum (Advanced / Query / Blocked / Done), generic over block type B (default BlockHash). Drivers now fetch block data by height instead of resolving candidate BlockIds.
• Added: BlockQueries, a request/resolve tracker, with resolve_candidates and the BlockCandidateResolution enum.
• Added: median_time_past(height, block_time), a free helper computing the BIP-113 median-time-past with the timestamp source supplied by closure. CheckPoint::median_time_past and the canonical-view MTP computation both delegate to it.
• Removed: the ChainRequest / ChainResponse type aliases.

bdk_chain

• Added: median-time-past on canonical views — CanonicalTx::mtp and Canonical::tip_mtp, opted into via CanonicalViewTask::with_mtp() and LocalChain::canonicalize_with_mtp().
• Changed: CanonicalTask and CanonicalViewTask now implement the poll-based ChainTask (previously ChainQuery) and take a generic block-type parameter B.
• Changed: LocalChain::canonicalize (the generic task runner) is renamed to LocalChain::run_task, and LocalChain::canonical_view (the high-level helper) is renamed to LocalChain::canonicalize.

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API

[codecov]

Codecov Report

❌ Patch coverage is 92.44851% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.71%. Comparing base (6d03fc3) to head (5dc4b51).

Files with missing lines	Patch %	Lines
crates/chain/src/canonical_view_task.rs	90.76%	10 Missing and 2 partials ⚠️
crates/chain/src/canonical.rs	85.41%	7 Missing ⚠️
crates/chain/src/canonical_task.rs	93.10%	5 Missing and 1 partial ⚠️
crates/chain/src/local_chain.rs	93.75%	2 Missing and 1 partial ⚠️
crates/core/src/block_queries.rs	97.24%	3 Missing ⚠️
crates/chain/src/indexed_tx_graph.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2139      +/-   ##
==========================================
+ Coverage   78.65%   78.71%   +0.05%     
==========================================
  Files          30       31       +1     
  Lines        5909     6107     +198     
  Branches      279      279              
==========================================
+ Hits         4648     4807     +159     
- Misses       1185     1227      +42     
+ Partials       76       73       -3     

Flag	Coverage Δ
rust	78.71% <92.44%> (+0.05%)	⬆️

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[oleonardolima]

@evanlinjin the PR dependency is solved, #2038 landed on master! 🚀 🚀
You can rebase this one, though I think just resetting and cherry-picking the last commits will be easier (sorry for the huge conflicts 😅).

[evanlinjin]

@oleonardolima We should have done this in one go in #2038. Now reviewers need to review 2 large refactor PRs. 😢

[oleonardolima]

@oleonardolima We should have done this in one go in #2038. Now reviewers need to review 2 large refactor PRs. 😢

Oh, agree! Though #2038 was long overdue and getting too convoluted, I think having this follow-up won't be that bad.

I'll make this review my top priority, let me know when it's ready.