fix(opencode): let a pinned websearch provider enable the tool for any provider

[sahiljagtap08]

Issue for this PR

Closes #44307

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The websearch tool is stripped from the tool list for every provider that is not opencode or opencode-go, unless OPENCODE_ENABLE_EXA or OPENCODE_ENABLE_PARALLEL is set. OPENCODE_WEBSEARCH_PROVIDER already pins the search backend, but it did not count as an enablement signal, so custom and local providers (Ollama, LM Studio, anything openai-compatible) could pin a backend and still never receive the tool.

This makes a pinned OPENCODE_WEBSEARCH_PROVIDER=exa|parallel also enable the tool, which gives custom providers one documented env var to turn websearch on. The env read moves out of selectWebSearchProvider into RuntimeFlags next to the existing exa and parallel flags, so validation happens in one place and the function is testable without touching process.env. Defaults are unchanged: without any of the three env vars, only opencode and opencode-go get the tool.

Also updates the tools doc, which only mentioned OPENCODE_ENABLE_EXA and did not mention that this works for any provider.

How did you verify your code works?

Extended the existing websearch tests: webSearchEnabled returns true for non opencode providers when a backend is pinned and stays false otherwise, and selectWebSearchProvider honors the pinned backend over the exa and parallel flags. bun test test/tool/ in packages/opencode: 341 pass. Typecheck and oxlint are clean.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Enough1122]

AI code review — automated review for reference, author can ignore or act on any point.

Good refactor — moving the OPENCODE_WEBSEARCH_PROVIDER parse out of process.env peeking at call time into validated runtime flags fixes both the "pinned provider doesn't enable the tool" bug and the hidden global read, and the precedence (pinned > parallel > exa) is sensible and tested. Three small points:

1. The env-validation branch lost all coverage. The rewrite turned the previous env-based test into a pure-function test (packages/opencode/test/tool/websearch.test.ts:18-22), which no longer exercises the actual parsing added in packages/opencode/src/effect/runtime-flags.ts:40-42 — i.e. that an invalid value like OPENCODE_WEBSEARCH_PROVIDER=bogus maps to undefined (tool stays hidden) rather than throwing at flag construction. That mapping is exactly where a regression would bite users; a one-case test against the flag layer would cover it.

2. Docs: mention credentials. packages/web/src/content/docs/tools.mdx:~260-268 now advertises pinning for any provider including Ollama/LM Studio, but exa still needs its API key configured; a half-sentence pointing to wherever keys are documented will save local-model users a confusing failure on first call.

3. Minor: the flag is snapshotted when the runtime layer is built, whereas the old code re-read the env each call. Irrelevant for CLI usage, just noting the semantics change for anything embedding opencode as a library.

Otherwise clean and ready.