fix(pipeline): fall back to PATH lookup for cocoindex binary

[HumanBean17]

Summary

• cocoindex_bin() used Path(sys.executable).parent / "cocoindex", which assumes a venv layout where cocoindex sits next to the Python interpreter
• When installed via pip install --user, scripts land in ~/Library/Python/X.Y/bin/ while sys.executable points to the framework Python elsewhere (e.g. /Library/Frameworks/Python.framework/...), causing "cocoindex not found next to Python" errors
• Adds shutil.which("cocoindex") as a fallback after the original sys.executable-relative check

Test plan

• 620 passed, 9 skipped — existing test suite unchanged
• Manual: pip install --user on macOS, confirm java-codebase-rag init finds cocoindex via PATH

🤖 Generated with Claude Code

[HumanBean17]

Review: fix(pipeline): fall back to PATH lookup for cocoindex binary

Clean, well-scoped fix. The fallback chain is sensible — venv-relative first (fast, no PATH search), then shutil.which, then return the original candidate so downstream errors still work.

One suggestion (non-blocking)

Both call sites that consume cocoindex_bin() check exe.is_file() and produce the error:

"cocoindex not found next to Python: {exe}"

After this change, the binary might come from PATH (e.g. ~/Library/Python/3.12/bin/cocoindex), so "next to Python" is misleading. Consider updating to just:

"cocoindex not found: {exe}"

at lines 120 and 182. Minor polish — safe to merge as-is.