DRIVERS-3529 Add env.agent to handshake metadata for agentic client identification

[aclark4life]

Summary

Adds a client.env.agent field to the MongoDB Handshake metadata so that agent-mediated usage of MongoDB (AI coding assistants / agentic clients such as Claude Code, Cursor, Gemini CLI) can be distinguished from direct human usage. See DRIVERS-3529.

Changes

• Add env.agent to the client document structure.

• New Agent subsection defining detection via an ordered environment-variable table (first populated match wins):

  Order	Env var	client.env.agent value
  1	AI_AGENT	raw value
  2	AGENT	raw value
  3	CLAUDECODE	claude-code
  4	CURSOR_AGENT	cursor
  5	GEMINI_CLI	gemini-cli
  6	CODEX_SANDBOX	codex
  7	AUGMENT_AGENT	augment
  8	OPENCODE_CLIENT	opencode

  Generic vars (AI_AGENT/AGENT) report their raw value; the six known agents report a fixed label. Generic wins over known. "Populated" = present with a non-empty value. Omitted entirely when none are populated; determination MUST NOT raise a user-visible error.

• Preserve env.agent alongside env.name in the size-limit truncation cascade.

• Add prose Test 2: Test that agent metadata is properly captured (renumbers the auth test to Test 3).

Open questions for reviewers

• Fixed label taxonomy: chose kebab-case (claude-code, gemini-cli) for a normalized, tool-agnostic taxonomy per the design memo. Product Analytics owns the taxonomy — happy to switch to the raw env-var forms if preferred.

Notes

Per the aligned approach with @jkovacs, v1 targets the top-traffic drivers (Node, Python). The other language tickets are split from DRIVERS-3529 and blocked on this spec change. PyMongo implementation is tracked under PYTHON-5929.

---

Please complete the following before merging:

• Is the relevant DRIVERS ticket in the PR title?
• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded clusters).