CallbackSet API

[luisorofino]

What does this PR do?

Refactors the ReAct callback system from a single ReActCallback Protocol (requiring all 4 methods) into a CallbackSet decorator-based registry where observers register only the events they care about.

Four new/changed source files:

• react/types.py (new) — ReActResult moved here to avoid a circular import
• react/callbacks.py (new) — four per-event callable Protocols + CallbackSet
• react/process.py (changed) — drops ReActCallback, uses callback_sets: list[CallbackSet], dispatch loops collapse to single fire_* calls
• tests for callbacks are also added

This PR also changes the name of the file client.py to anthropic_client.py (as well as its test), for it to be more descriptive. In that file, Anthropic-specific logic will live isolated from the rest of the model-agnostic files.

Motivation

The old ReActCallback Protocol forced every observer to implement all 4 lifecycle methods even when it only cared about one. The new CallbackSet API lets callers register only what they need using decorator syntax:

cb = CallbackSet()

@cb.on_complete
async def log_done(result: ReActResult) -> None:
    print(f"Done in {result.iterations} iterations")

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

❌ Patch coverage is 97.03390% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (loa/openmetrics-ai-gen@1e530c8). Learn more about missing BASE report.

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