feat: add tags resource to mcp server

[yelizhenden-mdb]

Proposed changes

Jira ticket: CLOUDP-393142

Purpose: Lists all operations belonging to a given tag in a loaded spec — giving an agent a focused, actionable view of one area of the API without loading the entire spec.

Key design decisions:

• Error on unknown tag — agents get clear feedback rather than a silent empty list
• Case-sensitive tag matching — mirrors the OpenAPI spec exactly
• URI decoding is transparent — agents use tag names as they appear in the spec (Flex Clusters); the server handles percent-decoding automatically
• Sorted by path then method — deterministic output regardless of spec ordering

Checklist

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works

Changes to Spectral

• I have read the README file for Spectral Updates

Further comments