Add generic per-bidder static param overrides for PBS

[prk-Jr]

Summary

• Adds bid_param_overrides to PrebidIntegrationConfig — a HashMap<String, Json> that lets operators enforce static per-bidder parameters on every outgoing Prebid Server request, regardless of zone context.
• These overrides are applied before the existing zone-specific bid_param_zone_overrides, giving operators a reliable way to inject server-side bidder IDs that client-side Prebid config cannot override.
• Refactors the duplicated inline merge logic (previously copy-pasted in the zone-override path) into a shared merge_bidder_param_object helper.

Changes

File	Change
crates/trusted-server-core/src/integrations/prebid.rs	New bid_param_overrides field on PrebidIntegrationConfig; merge_bidder_param_object helper; apply static overrides before zone overrides in OpenRTB construction; tests
crates/trusted-server-core/src/settings.rs	Env var override test for TRUSTED_SERVER__INTEGRATIONS__PREBID__BID_PARAM_OVERRIDES
trusted-server.toml	Commented example for the new field
.env.example	Example env var usage

Closes

Closes #617

Related: #339, #383, #384

Test plan

• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed