providers: scope error event emissions

[yisding]

Problem

Provider error publication still owns fire-and-forget tasks through a local set and drains them with gather(return_exceptions=True). That ownership is invisible to the Session runtime tree, and STTBase reaches across implementations with a duck-typed getattr drain.

Changes

• attach STT/TTS ProviderErrorEmitter work to named Session RuntimeScope children
• use a closing local scope for standalone provider usage
• add an explicit provider-error drain hook and remove the duck-typed STT reach-across
• add RuntimeScope.drain(..., suppress_errors=True) to preserve the reviewed log-and-drop emission policy without cancelling sibling work
• split the dependency-free credential predicate into a leaf module to preserve import layering
• shrink the source ratchet by one raw task spawn and one gather(return_exceptions=True) site

Validation

• just check — 8519 passed, 308 skipped
• focused provider/runtime/Session/credential suite — 271 passed, 2 deselected
• source and teardown-budget ratchets
• one unrelated telephony 200 ms timing test expired on the first parallel full-suite run; the exact node passed in isolation, and the complete just check rerun passed

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[chatgpt-codex-connector]

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