fix: stop non-streaming generation at the first stop sequence (+ contract tests)

[solderzzc]

Part of #128 (Tier 2, contract tests). Found while auditing finish_reason / usage across the four response paths after #227.

Problem

The streaming handlers end generation as soon as a stop sequence matches. handleChatNonStreaming and handleTextNonStreaming did not: they generated to EOS or max_tokens and trimmed the text afterwards. The same request therefore spent more compute and reported a larger completion_tokens when it was not streamed.

Change

Both non-streaming loops now stop at the first stop sequence, using the same windowed checkStopSequences scan as the streaming paths. .info does not arrive on that exit, so completion_tokens is the chunk count, the same trade-off the streaming path already documents.

Tests (tests/test-contract.sh)

• Test 11: finish_reason is length when max_tokens ends a chat completion (non-streaming and streaming) and a streamed text completion. These paths were already correct; this guards them. The non-streaming /v1/completions case is left to Non-streaming /v1/completions reports finish_reason: "stop" when max_tokens was hit #227.
• Test 12: for chat, completion_tokens after a stop sequence agree between streaming and non-streaming.
• Test 12b: for non-streaming /v1/completions, a stop sequence reduces completion_tokens.

Both 12 and 12b first check that the unconstrained reply contains the stop word, and report SKIP otherwise, so they test the server and not the model.

Verification

Qwen2.5-0.5B-Instruct-4bit, release build, Xcode 26.3 (Swift 6.2.4):

	Test 12	Test 12b	Total
main (7e219b4)	FAIL, 38 vs 14 tokens	FAIL, 60 vs 60 tokens	17 passed, 2 failed, 2 skipped
this branch	pass, 14 vs 14	pass, 29 vs 60	19 passed, 0 failed, 2 skipped

The 2 skips are the existing tool-call and JSON-completeness checks, which the tiny model cannot satisfy. Unit suite: 435 tests, 9 skipped, 0 failures.

Not changed

Non-streaming chat reports tool_calls over length when a tool call was parsed, while the streaming path reports length. Which should win is a behaviour decision, so it is left alone here.

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