fix: do not omitempty ReadOnlyHint in ToolAnnotations

[pvlbzn]

ReadOnlyHint is a bool with omitempty, so the zero value (false) is indistinguishable from unset, and drops out of marshaled JSON.

The current behavior causes an issue with the OpenAI MCP app submission because they require explicit hints.

Consumers that explicitly set ReadOnlyHint: false on write tools lose the field on the wire. Removing omitempty ensures false is always serialized, which matches the MCP spec default.

[maciej-kisiel]

I would also suggest adding a new point to internal/docs/rough_edges.src.md (don't forget to run go generate ./...) that all hints should be been specified as *bool for full control of whether they are sent on the wire or not.

[pvlbzn]

Thanks for the review and suggestions @maciej-kisiel @guglielmo-san. Updated the branch 👍

[maciej-kisiel]

One last thing: I believe this is a behavior change that may result for example in breaking tests of our users. Given that, I would introduce a MCPGODEBUG flag to be able to temporarily revert to previous behavior. Please look for usages of mcpgodebug on how to set this up.

[pvlbzn]

Great call @maciej-kisiel , implemented compatibility mechanism according to mcpgodebug, and updated documentation.

[maciej-kisiel]

Please fix the staticcheck error.

[pvlbzn]

Thank you, team 🙌