feat: add Serply search provider - #507

I have read the CONTRIBUTING.md file.

YES

What kind of change does this PR introduce?

Feature: a new app connector, proposed in #506.

What is the current behavior?

There is no search/SERP connector in the app catalog, so agents that need live Google search results cannot use a workspace-managed key through the gateway.

Closes #506

What is the new behavior?

Adds a Serply connector following the existing Datadog pattern:

• packages/api/src/apps/serply.ts: app definition with an api_key connection method (single apiKey field)
• packages/api/src/apps/app-permissions/serply.ts: one read-only tool group covering web search, news, scholar, and video results on api.serply.io (all four endpoint paths verified against the live API)
• apps/gateway/src/apps.rs: AppProvider entry using AuthStrategy::None plus credential_headers to inject the stored key as a raw X-Api-Key header, the same mechanism the Datadog provider uses for DD-API-KEY, so no new gateway auth strategy is needed. Four unit tests mirror the Datadog ones (host match, no auth header injected, credential header mapping, no false positives on serply.io/www.serply.io)
• icon and a Cloud & Data entry in app-categories.ts

Serply is fully optional: nothing changes for workspaces that do not connect it.

Checks run locally per CONTRIBUTING:

• pnpm build passes
• pnpm check passes (includes cargo clippy, cargo fmt --check, tsc, prettier, and the vitest suite)
• cargo test in apps/gateway: 799 passed, including the 4 new Serply tests

Additional context

API docs: https://serply.io/docs

Disclosure: I work with Serply. Happy to adjust scope, naming, or drop this entirely if it is not a direction you want for the project.