[Official SDK Migration] Migrate session and metrics hooks

[Shweta-Deshpande]

What changed

Adds the last piece of the official go-sdk migration: session hooks and metrics hooks, replacing the mark3labs server.Hooks equivalents for the official SDK path.

• pkg/mcp-official/server.go: Added WithOnSession, a new Option that runs once per session, right after the client finishes initializing. This is the go-sdk's only per-session hook point.
• cmd/terraform-mcp-server/init.go: Wired WithOnSession into getOfficialStreamableServer to pre-create the session's TFE/HTTP clients on connect, then wait in the background and clean them up (plus rate-limit state) on disconnect. This replaces the old AddOnRegisterSession/AddOnUnregisterSession hooks.
• pkg/mcp-official/tools/middleware/metrics.go (new): A middleware that records tool-call counts, latency, errors, and client type — the go-sdk equivalent of the mark3labs AddAfterInitialize/AddBeforeCallTool/AddAfterCallTool hooks used for metrics. It reads client info straight from the session instead of needing a separate cache.
• pkg/client/metrics.go: Split RecordToolCall into a common RecordToolCallByName, so both the mark3labs hooks and the new go-sdk middleware share the same recording logic.
• Added tests for both the new metrics middleware and the session cleanup lifecycle (session_lifecycle_test.go), the connecting and disconnecting flow, that were previously not covered.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.