fix(agents): refuse a stream under another run's id or a second time

[justintime4tea]

StreamingAgent::stream on a built Agent streamed under the id it began with whatever request_id it was given, and a second call reused the already-cancelled run. The run-id layer (#796) had given the orchestration factory the same mismatch. Both now claim the run's one stream through StreamClaim: a request_id that isn't the run's id (in the run's own spelling), or a second call, returns a run whose stream yields one StreamRefused and does nothing. The id is checked before the claim is taken. Only the trait method claims, so the coordinator's transient retry (stream_chat_with_depth) still re-streams. An OrchestratorFactory fixes its run id when built, minting one if the config names none. StreamingAgent::run_id() returns the run an agent streams, so a caller holding only the trait object a builder returns can stream an agent built with run_id: None; it replaces the inherent run_id on Agent and the factory. StreamClaim is public so a StreamingAgent outside aura keeps the same contract; the test mock does, and its callers stream under its run_id(). The trait doc states the contract and the doc examples build with the id they stream under; the server paths (chat, A2A, Slack) already did.

Verification

• cargo +nightly fmt --check, cargo clippy --workspace --all-targets --all-features -- -D warnings, and cargo test --workspace (2,588 passed) on this layer.

Fixes: GH-790
Ref: GH-778

[greptile-apps]

Confidence Score: 5/5

[Critical impact] The PR appears safe to merge; no actionable new issue was found.

Summary

The PR makes each agent accept one stream under its own run id.

• StreamClaim rejects a wrong id without taking the stream.
• StreamingAgent::run_id() exposes the id through the builder’s returned trait object.
• OrchestratorFactory fixes its id when built.
• The mock now follows the same contract, addressing the previous unnumbered finding.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A["StreamingAgent::stream"] --> B{"Id matches?"}
  B -->|No| R["Return one StreamRefused"]
  B -->|Yes| C{"Stream already claimed?"}
  C -->|Yes| R
  C -->|No| D["Claim and start stream"]
  E["Coordinator retry"] --> F["stream_chat_with_depth"]
  F --> G["Stream without another claim"]

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