gopls/internal/mcp: add go_rename tool

[arjunmahishi]

This tool uses the LSP server's Rename method to rename Go symbols
(variables, functions, types, etc.) at a specified position. It supports
both preview mode (dryRun: true, default) and actual application
(dryRun: false) of changes across the workspace.

• Add renameTool() and renameHandler() to the MCP server
• Support dryRun parameter: true for preview, false to apply changes
• Add TestMCPRename to test the tool for both dryRun modes
• Update MCP instructions with go_rename usage examples for both modes

Fixes golang/go#74887

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 3f86730) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/tools/+/694036.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
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[gopherbot]

Message from Arjun Mahishi:

Patch Set 1:

(1 comment)

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