fix(mixin): stop failover after partial output

[Whxuan0701]

Closes #798

What

• Treats the first visible MixinSession chunk as the response commit point.
• Keeps transparent failover for failures that occur before any visible output.
• After a committed response fails, emits the terminal error once, drains the current generator for its returned content blocks, and selects the backup only for the next request.
• Adds deterministic tests for pre-output failover, post-output failure, generator completion, and next-request routing.

Why

The old retry loop could return A-part + A-error + B-complete in one assistant response. Besides confusing users, mixing two independently generated continuations can corrupt structured output and tool calls.

Validation

• Parent baseline f6e5657: deterministic reproduction calls both nodes and yields ['A-part', '!!!Error: reset', 'B-complete'].
• New focused suite: 4 passed on Python 3.10.20, 3.12.13, and 3.13.14.
• Full non-release frontend suite: 244 passed.
• Release qualification suite: 35 passed.
• python -m py_compile llmcore.py and git diff --check passed.

Scope

This does not change which provider/SSE errors are retryable inside a node. It only prevents cross-node failover after output has already been committed.

[Whxuan0701]

Implementation and deterministic regression tests are ready in this draft. The tests cover pre-output failover, post-output commit semantics, generator completion, and routing on the next request.