fix(gtpv2c): support cause-only Create Bearer rejection

[VerifiedOrganic]

Summary

• permit a rejected S2b Create Bearer Response to omit Bearer Context IEs under TS 29.274 clause 6.1.1
• preserve mandatory contexts for accepted and partially accepted responses
• keep exact sequence and full-count correlation when a rejection does include contexts
• document the public behavior change

Closes #653.

Validation

• cargo fmt --all -- --check
• git diff --check
• CARGO_BUILD_JOBS=2 cargo test --locked -p opc-proto-gtpv2c --all-features
• CARGO_BUILD_JOBS=2 cargo test --locked -p opc-proto-gtpv2c --test dedicated_bearer (32 passed)
• CARGO_BUILD_JOBS=2 cargo clippy --locked -p opc-proto-gtpv2c --all-targets --all-features -- -D warnings
• two read-only adversarial review passes; no unresolved findings

Checklist

• Relevant formatting, whitespace, tests, and clippy gates pass locally.
• Commit is signed off.
• Changelog and Rust API documentation are updated.
• Regression and negative tests are included.
• No new dependencies.
• No RFC/ADR change is required for this standards-conformance correction.