feat(mcp): refine login tool and address security verification (#10953)

### Description
Refined the login MCP tool description, input schema, and return instructions to enforce displaying the Session ID to the user for security verification (resolving phishing risks). Also aligned the behavior with the CLI tool to skip initiating a new session if already logged in and return the authenticated account email, and added a `reauth` parameter to force reauthentication.

Fixes #10942

### Scenarios Tested
- Run `firebase_login` tool without `authCode` when already logged in: Returns "Already logged in as ...".
- Run `firebase_login` tool without `authCode` but with `reauth: true` when already logged in: Initiates the login flow.
- Run `firebase_login` tool without `authCode` when not logged in: Returns login URI, sessionId, and critical security verification instructions.
- Run `firebase_login` tool with `authCode`\` and a pending session: Completes login successfully.

### Sample Commands

N/A