test(perplexity): add cassette coverage

[gold-silver-copper]

Summary

• add Perplexity provider cassette replay/record support
• add recorded cassettes for completion, Perplexity-specific options, streaming, context, and chat history
• register Perplexity fixtures in cassette safety checks

Verification

• cargo fmt
• RIG_PROVIDER_TEST_MODE=record cargo test -p rig --all-features --test perplexity perplexity::cassette -- --nocapture --test-threads=1
• RIG_PROVIDER_TEST_MODE=replay cargo test -p rig --all-features --test perplexity perplexity::cassette -- --nocapture --test-threads=1
• cargo test -p rig --all-features --test perplexity cassette_safety -- --nocapture
• cargo test -p rig --all-features --test perplexity -- --nocapture --test-threads=1

[gold-silver-copper]

Added a follow-up commit covering Perplexity-specific #2040 migration pain points with live cassettes:

• text-only content-part arrays flatten to plain string content
• tool-exchange history is stripped/remerged for Perplexity's strict alternation
• unsupported tools + multi-name ToolChoice::Specific are dropped before validation
• output_schema is dropped instead of sent as response_format

Verification rerun:

• RIG_PROVIDER_TEST_MODE=record cargo test -p rig --all-features --test perplexity perplexity::cassette::migration_pain_points -- --nocapture --test-threads=1
• RIG_PROVIDER_TEST_MODE=replay cargo test -p rig --all-features --test perplexity perplexity::cassette -- --nocapture --test-threads=1
• cargo test -p rig --all-features --test perplexity cassette_safety -- --nocapture
• cargo test -p rig --all-features --test perplexity -- --nocapture --test-threads=1