fix(openai): normalize streamed stop responses with tool calls

[akirarat]

OpenAI-compatible streams can return tool calls with finish_reason: "stop". Previously, the adapter published completed tool calls followed by a normal stop. The streaming agent then recorded those calls without dispatching them or continuing. Malformed or empty arguments could also pass through this path as a successful stop.

Validate all accumulated tool arguments before finalizing this response shape. Normalize valid calls to FinishReasonToolCalls; reject malformed or empty arguments with the existing retryable incomplete-stream error before publishing ToolInputEnd, ToolCall, or a successful finish. The existing guards for truncated and abnormal finishes remain in place.

The new mock-stream regressions cover valid, malformed, and empty arguments, plus agent-level dispatch and continuation. Applying only these tests to the base commit reproduces the failures; all pass with this change.

Validation:

• go test ./... -count=1 -timeout=10m
• go build ./...
• golangci-lint run using CI's v2.13.2: zero issues
• Formatted with gofumpt; git diff --check passed

Additional check: standalone builds in examples and examples/structured-outputs request go mod tidy before compilation. Both failures also reproduce on unchanged main; this patch does not modify their module files.

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features). Not applicable: bug fix.