mcp: reject relative paths in tool handlers

[Shrey327]

Description

Adds validatePath() to the MCP server that rejects non-absolute path arguments before shelling out to the func CLI. Since cmd.Dir is never set in the executor, relative paths silently resolved against the MCP server's working directory instead of the user's project turning a wrong directory failure into a silent misbehavior. This change turns that into an actionable error, matching the guidance already documented in instructions.md.All existing tests updated to use t.TempDir() and a TestValidatePath unit test added.

[knative-prow]

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This pull-request has been approved by: Shrey327
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[linux-foundation-easycla]

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[knative-prow]

Welcome @Shrey327! It looks like this is your first PR to knative/func 🎉

[knative-prow]

Hi @Shrey327. Thanks for your PR.

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