fix: use correct precision format for v2 write API

[lqleon1214]

Closes #

Proposed Changes

Bug: Precision::as_str() returns the v3 format (nanosecond, microsecond, millisecond, second), but when the v2 API is selected (use_v2_api = true, targeting /api/v2/write), the server expects the short format (ns, us, ms, s). Previously send_lp always used as_str() regardless of v2 or v3, so the precision parameter was sent in the wrong format for v2 writes and could be rejected or misinterpreted by the server.

Fix:

• Added Precision::as_v2_str() returning the v2 short-format strings
• Updated send_lp to select the correct precision format based on use_v2_api:
  • use_v2_api = true → as_v2_str() (ns/us/ms/s)
  • use_v2_api = false → as_str() (nanosecond/microsecond/millisecond/second)
• Added a unit test for as_v2_str() verifying all four precisions output correctly and round-trip through the v2 format

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional
• Sign CLA (if not already signed)