[PECOBLR-1928] Add AI coding agent detection to User-Agent header

[vikrantpuppala]

Summary

• Adds internal/agent package that detects 7 AI coding agents (Claude Code, Cursor, Gemini CLI, Cline, Codex, OpenCode, Antigravity) by checking well-known environment variables they set in spawned shell processes
• Integrates detection into InitThriftClient to append agent/<product> to the User-Agent header
• Uses exactly-one detection rule: if zero or multiple agent env vars are set, no agent is attributed (avoids ambiguity)

Approach

Mirrors the implementation in databricks/cli#4287 and aligns with the latest agent list in libs/agent/agent.go.

Agent	Product String	Environment Variable
Google Antigravity	antigravity	ANTIGRAVITY_AGENT
Claude Code	claude-code	CLAUDECODE
Cline	cline	CLINE_ACTIVE
OpenAI Codex	codex	CODEX_CI
Cursor	cursor	CURSOR_AGENT
Gemini CLI	gemini-cli	GEMINI_CLI
OpenCode	opencode	OPENCODE

Adding a new agent requires only a new constant and a new entry in knownAgents.

Changes

• New: internal/agent/agent.go — environment-variable-based agent detection with injectable env lookup for testability
• New: internal/agent/agent_test.go — 11 test cases covering all agents, no agent, multiple agents, empty values, and real os.Getenv
• Modified: internal/client/client.go — calls agent.Detect() when building User-Agent in InitThriftClient

Test plan

• internal/agent — 11 tests pass
• internal/client — all existing tests continue to pass
• Manual: verified User-Agent contains agent/claude-code when run from Claude Code via GODEBUG=http2debug=2

  http2: Transport encoding header "user-agent" = "godatabrickssqlconnector/1.10.0 agent/claude-code"
  

• Executed SELECT 1 successfully against dogfood warehouse with the new header

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds AI coding agent detection to the User-Agent header by checking for well-known environment variables set by 7 different AI coding agents. The detection uses an "exactly-one" rule to avoid ambiguous attribution when multiple agent environments overlap. The implementation mirrors the approach from databricks/cli#4287.

Changes:

• Added new internal/agent package with environment-variable-based agent detection logic and comprehensive test coverage
• Integrated agent detection into the User-Agent header construction in InitThriftClient

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/agent/agent.go	New package implementing AI coding agent detection via environment variables with injectable env lookup for testability
internal/agent/agent_test.go	Comprehensive test suite with 11 test cases covering all agents, edge cases, and real os.Getenv usage
internal/client/client.go	Integration of agent detection into User-Agent header construction, appending agent/<product> when detected

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