feat(types): ByteStrategy::Base64UrlUnpadded for RFC 7515 alphabets

[lightsofapollo]

Summary

• Adds a third [generator.types] byte variant, base64_url_unpadded, that swaps the inlined base64_serde helper's engine from STANDARD to URL_SAFE_NO_PAD (RFC 7515 §2). Per-field #[serde(with = \"base64_serde\")] attributes are unchanged — the alphabet is picked once at codec-emit time from the config, so the variant is opaque to call sites.
• Driven by gpu-cli portal-schema work: JWK thumbprints, DPoP, and JOSE-adjacent pubkey fields don't round-trip through the standard padded alphabet. Without this, format: byte had to be specced as plain string with the alphabet documented in field descriptions.
• Spec-global, not per-field. Specs that need to mix both alphabets in one file remain a follow-up (would need either two helper modules or a serde_with path discriminator).

What changed

File	Change
src/type_mapping.rs	New ByteStrategy::Base64UrlUnpadded variant; map_byte routes both Base64 variants through the same base64_serde codec name + TypeFeature::Base64.
src/generator.rs	Helper-emission block reads self.config.types.byte and substitutes the engine identifier inside the inlined codec; single ENGINE binding replaces three literal STANDARD references.
src/lib.rs	Re-exports ByteStrategy so external consumers can build custom configs.
tests/typed_scalars_test.rs	Two integration tests: byte_url_unpadded_emits_url_safe_no_pad_engine (new variant emits URL_SAFE_NO_PAD, no STANDARD) and byte_default_emits_standard_engine (locks the historical default).
src/type_mapping.rs (tests)	Two unit tests: codec-path reuse for the new variant and TOML parse of base64_url_unpadded.

Consumer usage

[generator.types]
byte = "base64_url_unpadded"

All format: byte fields in the spec then round-trip via URL-safe unpadded base64.

Test plan

• cargo build --lib clean
• cargo test — 51 binaries, 257 passed, 0 failed, 6 ignored
• cargo clippy --lib clean (pre-existing -D warnings failures in tests/oneof_untagged_tests.rs are unrelated)
• TOML key base64_url_unpadded round-trips through serde
• Default Base64 path still emits STANDARD (no regression)
• Downstream smoke: regenerate a portal spec with byte = \"base64_url_unpadded\" and confirm cargo check on the generated crate

Closes beads issue `openapi-generator-76h`.

🤖 Generated with Claude Code