fix(brave-search): reject blank queries

[tang-vu]

Why this matters

Both Brave proxy routes accept a whitespace-only q because non-empty strings are truthy. Those requests reach the paid upstream despite carrying no meaningful search terms, consuming quota and producing provider-dependent results.

Root cause: presence validation did not apply the same trimming used for returned result fields.

Behavioral invariant: missing and whitespace-only queries receive the existing 400 missing_query_param_q contract and never call Brave; valid queries are trimmed once and used consistently.

Overlap check

Searched open and closed upstream PRs for “brave search whitespace query” and reviewed PRs changing brave-search/proxy.mjs. Open PR #2998 validates malformed result entries and #2734 concerns caller authentication. Neither validates the query boundary; no matching closed PR was found.

Regression test

The loopback contract suite sends blank queries through real proxy processes for both /v1/search and enabled /search?format=json. It asserts the 400 body and verifies the stub-upstream request counter does not change.

Validation

• bash tests/test-brave-search-searxng-compat.sh — passed
• node --check extensions/services/brave-search/proxy.mjs — passed
• node --check tests/test-brave-search-searxng-compat.mjs — passed
• git diff --check — passed

Tradeoffs and rollback

Leading and trailing whitespace is normalized before forwarding, which matches user search semantics while preserving internal whitespace. The response error code remains backward-compatible. Rollback is one commit.