fix(agent): return message instead of raising ItemNotFoundError in get_lineage_paths_between

[psaikaushik]

Summary

When no lineage path exists between two assets, get_lineage_paths_between raises ItemNotFoundError. In an agent/ADK context this surfaces as an unhandled exception instead of a useful response.

Closes #16882

What Changed

datahub-agent-context/src/datahub_agent_context/mcp_tools/lineage.py

• get_lineage_paths_between() now catches ItemNotFoundError and returns a friendly message dict instead of raising
• Added _build_no_path_found_message() helper that returns a consistent response with message, source, target, pathCount=0, and empty paths
• Both code paths are covered: auto-discover (tries downstream then upstream) and explicit direction
• Updated docstring to document the new return shape for the no-path case
• Removed ItemNotFoundError from the Raises section of the docstring

datahub-agent-context/tests/unit/mcp_tools/test_lineage.py

• Replaced test_path_not_found_raises_error (which expected ItemNotFoundError) with test_path_not_found_returns_message that checks the message dict
• Added test_path_not_found_auto_discover_returns_message for the auto-discover code path

Before vs After

Before (raises, agent sees unhandled error):

ItemNotFoundError: No lineage path found between urn:li:dataset:A and urn:li:dataset:B
in either upstream or downstream direction

After (returns dict, agent gets a usable response):

{
  "message": "No lineage path found between urn:li:dataset:A and urn:li:dataset:B in either upstream or downstream direction.",
  "source": {"urn": "urn:li:dataset:A"},
  "target": {"urn": "urn:li:dataset:B"},
  "pathCount": 0,
  "paths": []
}

Notes

• Internal helpers (_find_upstream_lineage_path, _find_lineage_path) still raise ItemNotFoundError for control flow. Only the public-facing get_lineage_paths_between catches and converts them.
• No changes to get_lineage() which has different behavior and is not affected by this issue.