fix(rmcp): preserve elicitation requestedSchema $schema dialect

[latent-9]

What

ElicitationSchema (de)serializes through an internal ElicitationSchemaWire
bridge. The bridge had no field for the top-level $schema keyword, so a
dialect declared on an incoming requestedSchema was accepted but silently
dropped when the schema was re-serialized.

The 2025-11-25 protocol revision allows a requestedSchema to carry a
$schema dialect identifier, and ElicitationSchema::from_type already emits
one (schemars draft-07), so the value was being lost on any round-trip.

Change

• Add a schema: Option<Cow<'static, str>> field (serde rename = "$schema")
  to both ElicitationSchema and the ElicitationSchemaWire bridge.
• Thread it through the two From conversions.
• Add a with_schema setter, matching the existing with_title and
  with_description builders. The struct is #[non_exhaustive], so an
  external caller needs a setter to populate the field.

The field uses skip_serializing_if = "Option::is_none", so a schema without a
dialect serializes exactly as before. Adding a field to a #[non_exhaustive]
struct is semver compatible.

Tests

Three unit tests in elicitation_schema.rs:

• a $schema value survives a deserialize then serialize round-trip,
• a schema without a dialect emits no $schema key,
• the with_schema setter serializes the dialect.

Scope

This handles the $schema half of #1168. The other half (preserving unknown
property-level keywords) needs a data-model decision on where arbitrary
keywords should live, so I left it out of this change and kept the issue open
for it.

Refs #1168