Remove async-trait from extension contributors

[jif-oai]

Why

Extension contributors are registered behind dyn Trait objects, so native async fn/RPITIT methods would make these traits non-object-safe. Spell out the boxed, Send future contract directly so extension-api no longer needs async-trait while retaining the existing runtime model.

What changed

• add a shared ExtensionFuture alias and use it for asynchronous contributor methods
• migrate production and test implementations to return Box::pin(async move { ... })
• remove async-trait dependencies where they are no longer used, keeping it dev-only where unrelated test executors still require it

Behavior

No behavior change is intended. Contributor futures remain boxed, Send, dynamically dispatched, and lazily executed; cancellation and callback ordering stay unchanged.

Testing

• just test -p codex-extension-api (11 passed)
• affected extension crates (64 passed)
• targeted codex-core contributor tests (14 passed)
• just fmt
• just bazel-lock-update
• just bazel-lock-check

A broad local codex-core run compiled successfully but encountered unrelated sandbox and missing test-binary fixture failures; CI will run the full checks.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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