agents: own bridge stream races

[yisding]

Summary

• move the Llama and Remote Responses cancel-token/next-item races into short-lived named RuntimeTaskScopes
• add scoped support for SDK awaitables such as async-generator __anext__() objects while preserving explicit iterator teardown
• verify stable task names and cancellation cleanup, reduce the raw-task baseline from 20 to 16, and remove Llama's reviewed C901 complexity exception

Test evidence

• env UV_FROZEN=1 just check — pre-commit passed, mypy passed across 288 source files, 8,593 tests passed and 309 skipped
• focused runtime and bridge suite — 170 passed, 1 deselected
• broader credential-free Llama/Responses slice — 188 passed, 2 skipped, 674 deselected
• complexity, source-inventory, and teardown-budget guards — 19 passed

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