fix(websearch): retry transient failures and fall back to other provider

[DScoNOIZ]

Issue for this PR

Closes #50227

Type of change

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

What does this PR do?

I hit this myself: on flaky egress the Parallel search endpoint periodically dies with transport errors, and since the tool makes exactly one attempt per call (provider pinned per session), search stayed broken for the whole session.

Changes, in packages/opencode/src/tool/{mcp-websearch,websearch}.ts:

• call() retries transient failures (3 attempts, exponential backoff + jitter, the same Effect.retry idiom used elsewhere in the repo);
• the 25s timeout now fails instead of dying, so retries and the fallback can actually catch it (the tool boundary still dies via orDie if both providers fail);
• if the pinned provider fails, the call falls back to the other one and reports which provider was actually used;
• opt-in MCP handshake (OPENCODE_WEBSEARCH_HANDSHAKE=1) for providers that start rejecting session-less calls. Without the flag behavior is unchanged.

No retry is infinite: 3 attempts max, then fallback, then the error surfaces as before.

How did you verify your code works?

• bun typecheck clean in packages/opencode;
• bun test test/tool/websearch.test.ts: 10/10 pass;
• live HTTP probes against search.parallel.ai: plain stateless tools/call works, initialize handshake returns a session id (verified the exact request shapes the code sends).

Screenshots / recordings

N/A (no UI change).

Checklist

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

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍