Implement generic Prebid bid param override rules

[prk-Jr]

Summary

• Replaces the split Prebid bidder-param override runtime with one generic ordered override engine.
• Keeps bid_param_overrides and bid_param_zone_overrides as compatibility config, while adding canonical bid_param_override_rules for future config-driven overrides.
• Normalizes all override config into one validated rule list at startup and applies rules by exact bidder / zone match with shallow last-write-wins merges.

Changes

File	Change
crates/trusted-server-core/src/integrations/prebid.rs	Add BidParamOverrideRule, BidParamOverrideWhen, and BidParamOverrideEngine; normalize compatibility fields and canonical rules into one runtime path; apply rules during OpenRTB construction; update tests
crates/trusted-server-core/src/settings.rs	Add env var override test for TRUSTED_SERVER__INTEGRATIONS__PREBID__BID_PARAM_OVERRIDE_RULES
trusted-server.toml	Document canonical bid_param_override_rules and clarify that compatibility fields normalize into the same engine
docs/superpowers/plans/2026-04-08-prebid-generic-bid-param-override-rules.md	Add implementation plan used for this branch

Closes

Closes #617

Related: #339, #383, #384

Test plan

• cargo fmt --all -- --check
• cargo test -p trusted-server-core
• cargo clippy -p trusted-server-core --all-targets --all-features -- -D warnings
• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings

Checklist

• Changes follow CLAUDE.md conventions
• Uses log macros (not println!)
• New code has tests
• No secrets or credentials committed

[ChristianPavilonis]

Well-designed feature. The BidParamOverrideEngine with its canonical rule format and compatibility normalization from bid_param_overrides/bid_param_zone_overrides is a clean architecture. Good validation at startup via try_from_config and json_object_for_override. Test coverage is thorough — especially the engine unit tests and precedence ordering tests. A few suggestions below.

[ChristianPavilonis]

Well-designed PR — the unified rule engine is clean, well-tested, and the compatibility migration strategy is sound. Approving with two recommended fixes.

[aram356]

Summary

Replaces the split bid-param override paths with a single generic BidParamOverrideEngine that normalizes all three config surfaces into an ordered rule list at startup. Clean design, solid compatibility story, comprehensive tests.

Non-blocking

♻️ refactor

• Redundant validation-only engine build in PrebidIntegration::try_new: builds and discards the engine just for validation — same work is repeated in PrebidAuctionProvider::try_new (prebid.rs:233)

🤔 thinking

• deny_unknown_fields on BidParamOverrideWhen: strict is good, but adding new matcher fields later becomes a breaking config change (prebid.rs:189)
• Shallow merge semantics vs. nested override values: real-world bidder params can be nested objects — shallow merge replaces the entire value rather than deep-merging (prebid.rs:504)

⛏ nitpick

• parse_prebid_toml_result error message: "should be enabled" is misleading when None could also mean the section is missing entirely (prebid.rs:1627)

🌱 seedling

• Zone-only rules (no bidder): the engine requires at least one matcher, so "for all bidders in zone X" rules aren't possible yet — could be useful for zone-wide floor overrides

CI Status

• fmt: PASS
• clippy: PASS
• cargo test: PASS
• vitest: PASS
• integration tests: PASS
• browser integration tests: PASS