* fix(sse): trust finish_reason:length/max_tokens over the reasoning-ratio heuristic in response quality validation
A truncated response with empty content and reasoning_content present was
only rejected by validateResponseQuality() when reasoning consumed >=90%
of completion_tokens. A response truncated at a lower ratio (e.g. 63%)
passed through as "valid" even though the caller received no usable
content and finish_reason was explicitly "length" (or the alternate
"max_tokens" naming some providers use) -- an unambiguous truncation
signal the validator wasn't reading. Reproduced live against
nvidia/nemotron-3-super-120b-a12b: content:null, finish_reason:length,
reasoning_tokens 645/1024 (63%).
Trust finish_reason directly when it's reported, falling back to the
existing token-ratio heuristic only when it isn't. Does not affect the
deliberate-tiny-probe case (e.g. max_tokens:1 connectivity pings) --
those never produce reasoning_content, so the branch this change is in
doesn't run for them.
* docs(changelog): add fragment for #12262
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Co-authored-by: brick30llc-ctrl <admin@brick30.com>
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