Serialize MCP-supplied object to string for JSON schema flags - #36

The approach is correct and the tests are thorough. One docs typo to fix:

Typo in docs/schema.md — duplicate json:"foo" tag

The updated SomeJsonObject example has both Foo and Bar tagged with json:"foo":

Foo    string `json:"foo" jsonschema:"..."`
Bar    int    `json:"foo" jsonschema:"..."`  // should be json:"bar"

Bar should be json:"bar". As-is the example would silently produce incorrect JSON, which is a confusing doc for users trying to learn the feature.

The fix is correct and well-tested. One issue in docs/schema.md:

Bar    int `json:"foo" jsonschema:"Description of Bar field"`

The Bar field has json:"foo", which duplicates the tag from Foo. This should be json:"bar". Users who copy-paste this example will get a struct where two fields map to the same JSON key, causing silent unmarshaling bugs.

Also a minor note: globalFlagRegistry is shared global state with no cleanup between tests. The current tests happen to work because each case registers fresh metadata for the same tool name "foo" immediately before calling buildFlagArgs. If any test were added that doesn't set up its own registry entry before calling into code that reads it, it would silently inherit a previous test's state. Worth keeping in mind if the test suite grows.