[Platform] Normalize finish_reason metadata across all bridges

[chr-hertel]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	Fix #2194
License	MIT

Follow-up to #2276, which exposed the raw streamed finish_reason for the OpenAI-compatible bridges only.

• Adds FinishReason\FinishReason + FinishReason\FinishReasonCase, normalizing the divergent provider vocabularies (length / max_tokens / MAX_TOKENS / max_output_tokens) into STOP, LENGTH, TOOL_CALL, CONTENT_FILTER, STOP_SEQUENCE, OTHER while getRaw() keeps the provider value.
• Exposes it as finish_reason result metadata for buffered and streamed results across OpenAI, Anthropic, Gemini, VertexAI, Bedrock, Azure, Ollama, Cohere, Mistral, DeepSeek, Cerebras, Scaleway, Docker Model Runner, Perplexity, MiniMax, amazee.ai and Generic.
• Streams now emit the reason as the final MetadataDelta, after the deltas of the chunk that carried it (providers such as Mistral bundle the last content token with finish_reason).

$finishReason = $result->getMetadata()->get('finish_reason');

if ($finishReason?->is(FinishReasonCase::LENGTH)) {
    // truncated by the output token limit
}

Docs in docs/components/platform.rst, runnable example in examples/misc/finish-reason.php.

Known gap: the OpenAI Responses API signals a truncated stream with response.incomplete, which the bridge raises as an exception, so there is no result left to carry the reason. Left as-is; changing it is a behavior change beyond this PR.