Fix docs.rs build for xsd-parser-types when all-features is enabled

[Copilot]

xsd-parser-types docs were failing on docs.rs because docs.rs builds with all-features, which enabled serde-based derives without enabling serde's derive support. This PR aligns crate feature wiring so the docs build path matches enabled code paths.

• Root cause

  • #[derive(serde::Serialize, serde::Deserialize)] is conditionally enabled behind the crate’s serde feature.
  • The serde dependency was optional but did not enable serde’s derive feature, so derive macros were unavailable in docs.rs all-features builds.

• Change

  • Updated xsd-parser-types/Cargo.toml to enable derive support on the optional serde dependency:

    serde = { workspace = true, optional = true, features = ["derive"] }

• Impact

  • Restores docs.rs compatibility for xsd-parser-types under all-features.
  • Keeps existing feature model intact; only serde macro availability is corrected.

Closes #277

[Bergmann89]

@copilot For some reason the CI pipeline did not start. Run it again.

[Copilot]

@copilot For some reason the CI pipeline did not start. Run it again.

I reran CI by pushing commit 6e0c626, which triggered a new Main workflow run (#552). It still ends in action_required with no jobs started, so it appears to require manual approval in Actions before jobs can execute.