fix(ai-gateway): sanitize Perplexity tool messages

[chrarnoldus]

Summary

• remove unsupported tool-message names before sending Perplexity Kimi chat-completions requests
• remove unsupported top-level user metadata from Perplexity requests
• cover selective tool-name removal with a request-helper unit test

Testing

• pnpm --filter web test -- --runInBand apps/web/src/tests/openrouter-request-helpers.test.ts apps/web/src/lib/ai-gateway/providers/provider-definitions.test.ts
• pnpm --filter web typecheck
• pnpm exec oxlint --config .oxlintrc.json apps/web/src/lib/ai-gateway/providers/provider-definitions.ts apps/web/src/lib/ai-gateway/providers/openrouter/request-helpers.ts apps/web/src/tests/openrouter-request-helpers.test.ts
• pnpm exec oxfmt apps/web/src/lib/ai-gateway/providers/provider-definitions.ts apps/web/src/lib/ai-gateway/providers/openrouter/request-helpers.ts apps/web/src/tests/openrouter-request-helpers.test.ts
• git diff --check