fix(think): protect concurrent tool calls with a mutex

[dlanov]

Summary

Fixes #4568.

The Think tool stores thoughts in a shared slice without synchronization. When multiple think calls execute concurrently, appending to the slice and building the response can race, resulting in lost thoughts or inconsistent output.

This PR adds a mutex to protect both operations, preserving the existing output format and sequential behavior.

Changes

• Protect Think ToolSet's shared thoughts with sync.Mutex.
• Add a sequential test verifying the exact output format.
• Add a concurrency regression test with 64 simultaneous tool calls, verifying that no thoughts are lost or duplicated.

Validation

• Reproduced the original bug with Go's race detector, including lost thoughts.
• Targeted race tests passed across 20 shuffled runs.
• go vet passed.
• task lint passed with 0 issues.
• task build passed.
• task test passed (243 packages).

No dependencies or tool lifecycle behavior were changed.