fix(elicitation): preserve enumNames through ElicitationSchema serde round-trip

[abdouloued]

Fixes#903.

Problem

EnumSchema is #[serde(untagged)] with variants ordered Single → Multi → Legacy. SingleSelectEnumSchema is itself untagged with Untitled before Titled. UntitledSingleSelectEnumSchema did not have deny_unknown_fields, so a legacy payload like:

{"type":"string","enum":["opt1","opt2","opt3"],"enumNames":["Option One","Option Two","Option Three"]}

was silently matched by UntitledSingleSelectEnumSchema (which has no enumNames field). The array was dropped, and LegacyEnumSchema was never tried. On re-serialization, enumNames was gone.

A secondary issue: LegacyEnumSchema::enum_names lacked skip_serializing_if = "Option::is_none", so a Legacy with enum_names: None would serialize "enumNames": null.

Fix

• Add deny_unknown_fields to UntitledSingleSelectEnumSchema. When enumNames (or any unrecognised field) is present, serde rejects this variant and falls through to LegacyEnumSchema.
• Add skip_serializing_if = "Option::is_none" to LegacyEnumSchema::enum_names.

Two regression tests added:

1. test_legacy_enum_schema_roundtrip_preserves_enum_names — mirrors the exact repro from the issue; verifies that a full ElicitationSchema round-trip keeps enumNames intact.
2. test_legacy_enum_schema_no_enum_names_omits_field — verifies LegacyEnumSchema with enum_names: None does not emit "enumNames": null.

All existing elicitation tests continue to pass.

[michaelneale]

@abdouloued do you want to fix up the failures and we can take a look again?

[abdouloued]

@abdouloued do you want to fix up the failures and we can take a look again?

Pushed a fix. The failure appears to be a stale golden-file schema snapshot (tests/test_message_schema/server_json_rpc_message_schema.json). The deny_unknown_fields change makes schemars add "additionalProperties": false to UntitledSingleSelectEnumSchema, which the snapshot test caught.

Regenerated it with UPDATE_SCHEMA=1 cargo test -p rmcp --test test_message_schema --all-features — no other changes needed, verified locally that it passes.

Looks like the workflow run needs approval to kick off again on this push.

[DaleSeo]

Thanks for the fix, @abdouloued!