build(sessions): vendor session.proto and set up tonic-build [2/12]

[sushanb]

Summary

Second in the stack. Vendors google/bigtable/v2/session.proto and its transitive proto dependencies from googleapis/googleapis, and adds tonic-build under a new sessions Cargo feature so the session-family types (OpenSessionRequest, CloseSessionRequest, GetClientConfigurationRequest, GoAwayResponse, SessionRefreshConfig, and their inner messages) are generated at build time.

Stacked on #154 (design record). The branch is layered on top of sessions/design-record, so this PR's diff currently includes both the spec/plan commits from #154 and the proto changes from this PR. Once #154 merges, I'll rebase this branch onto main and the diff will collapse to just the proto changes.

Changes

• Vendored .protos under bigtable_rs/proto/google/{bigtable/v2,api,rpc,type}/ (verbatim from googleapis@main)
• build.rs compiles only session.proto — all types defined in imported .protos (data.proto, types.proto, request_stats.proto, feature_flags.proto, peer_info.proto, response_params.proto) are redirected via extern_path to their existing generated equivalents in googleapis-tonic-google-bigtable-v2 and googleapis-tonic-google-rpc. No duplicate type generation.
• Generated bindings exposed as bigtable_rs::session_proto (hidden from public docs; will be consumed by later PRs)
• sessions Cargo feature (default-off) — users who don't opt in pay zero protoc build cost
• Smoke test (tests/session_proto_smoke.rs) asserts every session-critical type is generated and that extern-path types resolve to the third-party crate's types

Rationale

The upstream googleapis-tonic-google-bigtable-v2 crate does not yet publish the session envelope types (verified against v0.36.0), but the .proto files are public in googleapis/googleapis. Vendoring + generating locally is the shortest unblocking path; when upstream regenerates, we can swap to their types with an extern_path change (no callers affected).

Test plan

• cargo build --package bigtable_rs (default features) — succeeds, unchanged behavior
• cargo build --package bigtable_rs --features sessions — succeeds, generates session_proto
• cargo test --package bigtable_rs --lib — 2/2 pass (existing tests)
• cargo test --package bigtable_rs --features sessions --test session_proto_smoke — 2/2 pass
• cargo fmt --check — clean

Stack

• Stacked on: docs(sessions): vendor Sessions specs and port plan [1/12] #154 (specs + plan)
• This PR (2/12): protos + tonic-build
• Next (3/12): Session lifecycle — state machine, hooks, heartbeat, GOAWAY handler
• ...9 more to follow per SESSION_PORT_PLAN.md

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.1%. Comparing base (2899ca5) to head (a3257b6).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #155     +/-   ##
=======================================
- Coverage   63.8%   59.1%   -4.6%     
=======================================
  Files          6       6             
  Lines        513     594     +81     
=======================================
+ Hits         327     351     +24     
- Misses       186     243     +57     

Flag	Coverage Δ
rust	59.1% <ø> (-4.6%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
bigtable_rs/src/lib.rs	100.0% <ø> (ø)

... and 1 file with indirect coverage changes

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