perf(tools): scan only the chunk for a collecting tool call's end tag

[spokvulcan]

Proposed changes

Fixes #624.

While collecting a tagged tool call, ToolCallProcessor and the text recovery scanner searched the whole buffered call for its closing tag on every chunk, which is quadratic in the call's length: a 13k-token write call spent seconds in that scan alone. Only the text a chunk appends can complete the closing tag, because an earlier occurrence was already in the buffer when the previous chunk was scanned. Both scanners now check the appended bytes plus the tag's overlap with what preceded them, through one helper on ToolCallFrameScanner, and run the structural frame scan only when that window holds the tag. The chunk that opens the frame still scans the whole buffer, which then holds at most the start tag and that chunk. Five tests in ToolCallProcessorLongCallTests: a 40k-character call streamed one character at a time parses in linear time, with and without declared tools, and the closing tag is found when it is split across chunks, when it arrives in the chunk that completes the start tag, and when another call follows it in the same chunk.

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