fix(go): make prompt.Desc() concurrency-safe

[arthu-rguo]

explanation:

• ActionDef.Desc() is careful to avoid mutating the receiver when resolving schemas.
• prompt.Desc() is not, as it mutates a deeply nested map without deep copying.
• Calling prompt.Desc() concurrently will produce a fatal (unrecoverable) Go runtime panic.

fix:

• Deep copy the nested map.

additional notes:

• While developing my application, I was surprised by this because I had assumed Name() and Desc() were concurrency-safe simple getters.
• I am guessing that this mutation was unintentional.
• If it was in fact intentional for memoization, I'm happy to close this PR (or pivot to documenting the behaviour).

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request updates the 'Desc' method in 'go/ai/prompt.go' to clone the metadata maps ('desc.Metadata', 'prompt', 'input', and 'output') before modifying them, preventing in-place mutation of shared metadata structures. There are no review comments, so I have no feedback to provide.

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