feat(blaze)!: remove legacy instance routes - #2585

Description

Blaze exposes sandbox lifecycle and guest operations only through /v1/sandboxes.

Blaze has not been released with a /v1/instances compatibility contract, so this change removes that HTTP namespace completely. Production routing contains no instance-namespace matcher or compatibility handler, and the long-lived documentation describes only the current sandbox routes. There is also no special regression test or response-precedence guarantee for the removed namespace; ordinary unregistered paths are handled by the server's general routing behavior.

The supported routes are:

• GET /v1/sandboxes
• POST /v1/sandboxes
• GET /v1/sandboxes/{id}
• DELETE /v1/sandboxes/{id}
• POST /v1/sandboxes/{id}/exec
• POST /v1/sandboxes/{id}/read
• POST /v1/sandboxes/{id}/write

Action-style reset, checkpoint, and destroy routes are not registered under the sandbox namespace. Canonical destruction remains DELETE /v1/sandboxes/{id}. Checkpoint capture and listing are delivered separately by #2472.

Existing JSON field names such as instance and instance_id, and the storage.instances_dir configuration key, remain unchanged because they describe data and storage rather than an HTTP namespace.

The change also removes the incomplete reset handler, the placeholder checkpoint handler, and server-state accessors that became unused with those handlers. Strict Clippy verifies that the resulting production code has no warnings or dead-code allowances introduced by this change.

Behavior before

• Lifecycle and guest operations were registered under both /v1/sandboxes and /v1/instances.
• The instance namespace exposed incomplete reset and placeholder checkpoint behavior.
• Documentation presented both namespaces to users.

Behavior after

• /v1/sandboxes is the only documented and registered management namespace.
• Sandbox create, list, get, delete, exec, read, and write retain their existing status codes and resource effects.
• Unsupported sandbox action routes remain unregistered.
• No production or test code assigns special semantics to /v1/instances.
• Checkpoint support can add its sandbox routes without preserving a second namespace.

Related issue

Closes #2577

Related checkpoint work: #2444, #2472

This pull request is based directly on current main and does not depend on #2293, #2296, or #2471.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code refactoring
• Performance improvement
• Other

Scope

• Core
• SDK
• Blaze
• Documentation
• Build/CI
• Other

Testing

Revision identity

• Public head: 8ec393c95aed8fb12f02583af4d6634160272f2a
• Head tree: 35fe2a74cc8575b75b654a6a9fa673e882662aad
• Parent and recorded main: 160297392b5f129dabd5accd6dd13214528832d5
• GitHub merge candidate: ce61858523bb1f527461429f7f7791f5a593dd46
• Merge candidate parents: 160297392b5f129dabd5accd6dd13214528832d5, 8ec393c95aed8fb12f02583af4d6634160272f2a
• Merge candidate tree: 35fe2a74cc8575b75b654a6a9fa673e882662aad

The public head and GitHub merge candidate are byte-identical source trees.

Linux validation

Validated on Linux x86_64 with Rust and Cargo 1.88, locked dependencies, offline Cargo access, a fresh source tree, a fresh Cargo home, and separate empty target directories for every major stage:

• cargo fmt --all -- --check
• locked metadata with default and all features
• workspace all-target builds with default and all features
• strict Clippy with default and all features
• serial workspace tests: 299 default and 314 all-feature tests passed
• strict rustdoc with default and all features
• repository documentation lint and link checks
• 8/8 exact sandbox-route, request-body, guest-operation, and response-boundary tests
• source tree and final Git tree unchanged after validation

Evidence archive SHA-256: b23a24206685304507a090bf770b1baca5abb9ca805c4c4e3b5510a821f1c9b4.

Hosted Blaze, documentation, website, commit-message, pull-request metadata, component, and CLA checks passed for the exact head. Codex reviewed 8ec393c95a and found no major issues. This pull request is ready for maintainer review.

Documentation

The English and Chinese Blaze README, user guide, and lifecycle design describe only the current /v1/sandboxes API. No migration note or version-log entry is included because Blaze has not been released with the removed namespace.