fix(protests,sled): match the API's protest enum casing, add naics_code, carry delisted_at in SLED shapes

[makegov-mark]

What

• Protests: enum casing now matches what the API returns. The rustdoc on ListProtestsOptions and ProtestRecord showed source_system as "GAO" / "COFC" and outcome as "sustained". The API returns lowercase sources (gao, cofc, sba_oha) and title-case outcomes (Denied, Dismissed, Withdrawn, Sustained, plus Granted, Remanded, Reversed, Vacated for SBA OHA). Filters are case-insensitive, so queries worked, but code comparing returned values against the documented examples would never match. Test fixtures now use the real casing too.
• Protests: SBA OHA is named as a source in the module doc, the model doc and docs/API_REFERENCE.md.
• Protests: new naics_code option on ListProtestsOptions, the one API protest filter the SDK lacked. Only SBA OHA size and NAICS appeals carry a code, so GAO and COFC records never match.
• Protests: agency rustdoc now says what the filter accepts (name, abbreviation or code, | for multiple values).
• SLED: delisted_at in both suggested opportunity shapes, matching the API's own list and detail defaults, with a note on how it drives status_reason = "delisted".

Tests

• list_protests_naics_code_emits: naics_code serializes to the naics_code query param. Fails to compile on main (no such builder method); passes here.
• suggested_opportunity_shapes_carry_delisted_at: both SHAPE_SLED_OPPORTUNITIES_* constants name delisted_at. Fails on main; passes here.
• just ci equivalent is green: cargo fmt --all -- --check, cargo clippy --workspace --all-features --all-targets -- -D warnings, cargo test --workspace.

Relationship to open PRs

• feat(contract-appeals): add the contract appeals resource #5 (contract appeals): the only overlap is the [Unreleased] section of CHANGELOG.md, a trivial both-sides-add conflict. shapes.rs and docs/API_REFERENCE.md auto-merge.
• Sync to the current Tango API: budget, DIBBS, exclusions, SBIR, detail GETs, filter parity (v0.2.0) #3 (API sync): already conflicts with main in CHANGELOG.md and crates/tango/src/lib.rs; this branch adds nothing beyond the same CHANGELOG.md section.

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