docs(mcp-genmedia): clarify image_size default wording + document no-default policy

[ghchinoy]

Fast-follow to #1733 (merged).

What

1. Wording fix — the image_size tool descriptions in mcp-gemini-go/main.go and mcp-nanobanana-go/main.go said "Defaults to 1K when unset", which reads as though the tool forces a 1K default. It does not: image_size is registered with no DefaultString, stays empty when unset, and is dropped by omitempty so the model applies its own default (currently 1K). Reworded to "When unset, the model's default (currently 1K) applies."
2. Policy note — added a short comment next to the image_size parsing in both handlers documenting that the no-default (passthrough) policy is intentional — the tool deliberately never asserts an image size the model may not support. This confirms/keeps the existing behavior rather than adopting mcp-imagen-go's DefaultString("1K").

Notes

• The two READMEs already carried the corrected wording, so no README change was needed.
• No behavior change — descriptions and a comment only.
• go vet ./... and go build ./... pass clean in both mcp-gemini-go and mcp-nanobanana-go.

Context: addresses recommendations #1 (wording nit) and #2 (confirm no-default policy) from the review of #1733. The registry-validation item (#3) is tracked as a separate issue.

[github-actions]

🤖 Hi @ghchinoy, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[ghchinoy]

Note for reviewer/merger: the build-and-test (MCP GenMedia Go CI) failure here is pre-existing and unrelated to this change. It has been red on main for 2+ days, including at the merge of #1733 itself, and fails in the first module it processes (mcp-avtool-go) with go: updates to go.mod needed; to update it: go mod tidy — before CI ever reaches the modules touched here.

This PR is docs/comment-only (a tool-description wording tweak + a policy comment) in mcp-gemini-go and mcp-nanobanana-go. Locally, go vet ./... and go build ./... pass clean in both. check-changes, cla/google, and lint-mcp-pr-title all pass.

Leaving this for a normal review/merge pass rather than admin-merging past the red (pre-existing) gate. The go mod tidy drift is worth fixing separately.

[github-actions]

📋 Review Summary

The Pull Request successfully implements a consistent "no-default" policy for the image_size parameter in both the mcp-gemini-go and mcp-nanobanana-go MCP tools. By keeping this parameter empty when unset, the tools leverage omitempty in the JSON serialization to drop it, allowing the underlying Gemini model to apply its own default. This is a robust and forward-compatible design that avoids forcing size parameters the target model might not support. The codebase changes are clean, precise, well-documented, and the tests pass perfectly.

🔍 General Feedback

• Consistent No-Default Design: Adopting a no-default passthrough design for image_size in Go-based MCP servers is an excellent choice that aligns perfectly with the corresponding Python implementation (models/gemini.py), ensuring unified behavior across different components of the repository.
• Accurate Tooling Descriptions: Updating the tool parameter definitions in main.go to explicitly mention that the model's default applies when unset ensures that API clients (such as Cursor or other agent clients) are correctly informed.
• Detailed and Clear Code Comments: The newly introduced comments in the handlers are extremely descriptive, explaining the technical why behind the change (the distinction from mcp-imagen-go) rather than just the what.
• Robust and Error-Free Code: The implementation is robust, free of any logic or static-analysis issues (both go vet and test suites pass 100% cleanly).