#25271 [rust] fix: oneOf discriminator with an enum-typed tag

Fixes #22344
Fixes #22018

When the variants of a oneOf declare the discriminator property as an enum (inline or $ref), the union gets #[serde(tag = "...")] while each variant struct still has the field. Serializing writes the tag twice, and deserializing a valid payload fails with missing field, since serde consumes the tag before the variant sees it.

For those unions the template now implements Serialize/Deserialize instead of using serde(tag):

• serialize: write the variant as is, adding the tag only if the variant left it unset
• deserialize: match on the tag and pass the whole object, tag included, to the variant

This is the same approach rust-axum takes. Variant structs don't change, and plain-string tags and allOf unions keep their current output.

This is an alternative to #22489, which stalled.

Still unsupported, same as on master: discriminators without mapping, and several mapping values pointing to the same schema.

Tests: RustClientCodegenTest#testOneOfEnumDiscriminatorKeepsTagInContent (3.0 and 3.1 specs), and tests/enum_discriminator_test.rs in the reqwest composed-oneof sample.

cc @frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

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Summary by cubic

Fixes oneOf discriminated unions whose discriminator tag is an enum (inline or $ref) so the tag is written once and valid payloads deserialize instead of failing with a missing-field error.

• For these unions, generates custom Serialize/Deserialize implementations instead of serde's tag attribute.
• Serialization writes the variant payload as-is, then always overrides the tag to the mapped value; deserialization matches the tag and passes the whole payload, tag included, to the variant.
• Plain string tags and allOf unions keep the existing serde(tag) output.
• Still unsupported: discriminators without mapping, and several mapping values pointing to the same schema.