feat(mcp): support templates and completion

[Nomadcxx]

Issue for this PR

Refs #28567. Stacked on #32936.

Type of change

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

What does this PR do?

Adds the resource templates / completion slice for MCP.

This adds resources/templates/list support through the MCP service, using the same pagination path as tools, prompts, and resources.

It also adds completion/complete plumbing for prompt refs and resource template refs. This is intentionally service-level only for now; it does not add dropdown UI or decide how completions should be surfaced in the TUI.

How did you verify your code works?

• bun test --timeout 30000 test/mcp/lifecycle.test.ts from packages/opencode
• bun typecheck from packages/opencode
• bun prettier --check packages/opencode/src/mcp/index.ts packages/opencode/src/mcp/catalog.ts packages/opencode/test/mcp/lifecycle.test.ts packages/opencode/test/session/prompt.test.ts packages/opencode/test/session/snapshot-tool-race.test.ts

Screenshots / recordings

N/A, no UI change.

Checklist

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

[Nomadcxx]

Folded the error-path test commit from the fork follow-up into this branch (commit 500619a). Tests: bun test --timeout 30000 test/mcp/lifecycle.test.ts → 43 pass.

[github-actions]

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