Add an MCP initialize readiness check

[Whxuan0701]

Summary

Agent harnesses and CI need a protocol-level readiness signal, but thv mcp list performs capability enumeration after initialize and is unsuitable for servers that intentionally expose no tools, resources, or prompts. A lightweight preflight should validate the MCP session handshake itself.

• Add thv mcp check with auto, SSE, and streamable HTTP transport support.
• Perform only transport setup, initialize, and connection close.
• Report negotiated protocol/server/capability metadata as text or JSON and return a non-zero exit code on failure.
• Keep check-specific flags isolated from sibling MCP commands and route output through Cobra writers.

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task -t Taskfile.focused.yml test, using the repository's -race and linker settings for ./pkg/mcp/client and ./cmd/thv/app)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Regenerated all CLI, Swagger, and Helm documentation with task docs, then verified the CLI documentation byte-for-byte against a fresh go run cmd/help/main.go output directory.

API Compatibility

• This PR does not break the v1beta1 API.

Changes

File	Change
pkg/mcp/client/client.go	Add a Probe API that returns initialize metadata and the selected transport without enumerating capabilities.
pkg/mcp/client/client_test.go	Exercise a real streamable HTTP MCP server and assert that the probe sends initialize but no list methods.
cmd/thv/app/mcp_check.go	Add the command with isolated flags, timeout handling, and text/JSON output through Cobra writers.
cmd/thv/app/mcp_check_test.go	Verify the five-second default survives parent-command construction and JSON is capturable through the command writer.
cmd/thv/app/mcp.go	Register the new command.
docs/cli/thv_mcp_check.md	Document the generated CLI workflow and flags.
docs/cli/thv_mcp.md	Link the new command from the generated parent command reference.

Does this introduce a user-facing change?

Yes. Users can now use thv mcp check --server ... --format json as an MCP-native readiness gate for CI, GitOps, and Agent harnesses.

Special notes for reviewers

The new probe intentionally does not call tools/list, resources/list, prompts/list, or any tool. Existing Connect callers retain their behavior and API.