fix(models): skip empty text parts in Anthropic message conversion

[MarcHuang168]

Link to Issue or Description of Change

Link to an existing issue:

Closes: #6982

Problem:

AnthropicLlm converts each Part into an Anthropic content block in _part_to_message_block. A part carrying only an empty text string (Part(text='')) matches no branch — if part.text: is false for '' — and falls through to the final raise NotImplementedError("Not supported yet: ..."), failing the whole model call with a misleading error.

ADK emits such parts itself: code_executors/code_execution_utils.py writes Part(text='') into the content when a code execution produces no output. The LiteLLM path already tolerates empty text parts, so the same conversation works via LiteLlm but fails via the native AnthropicLlm adapter.

Solution:

Add _is_empty_text_part() (a part whose only payload is an empty text, optionally with thought) and skip such parts in _content_to_message_param, logging at debug level. This mirrors the adapter's existing warn-and-skip handling of unsupported media in assistant turns, and Anthropic rejects empty text blocks anyway. Parts that carry other payload alongside empty text (e.g. a function_call) are unaffected, and the NotImplementedError fallback for genuinely unsupported part types is kept.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added in tests/unittests/models/test_anthropic_llm.py:

• test_content_to_message_param_skips_empty_text_part — an empty text part is dropped and the remaining text block is preserved.
• test_content_to_message_param_keeps_non_text_payload_with_empty_text — a part with text='' plus a function_call still produces a tool_use block.

Verified the first test fails without the fix:

$ git stash push -- src/google/adk/models/anthropic_llm.py
$ pytest tests/unittests/models/test_anthropic_llm.py -q -k empty_text
1 failed, 1 passed, 151 deselected
$ git stash pop

With the fix:

$ pytest tests/unittests/models/test_anthropic_llm.py -q
153 passed
$ pytest tests/unittests -q -n auto
13749 passed, 81 skipped, 26 xfailed, 2 xpassed

pre-commit run --files <the two changed files>: all hooks pass.

Manual End-to-End (E2E) Tests:

Ran the reproduction from #6982 (content_to_message_param on a Content with Part(text="run it") and Part(text="")). Before: NotImplementedError: Not supported yet: ... text='' .... After: {'role': 'user', 'content': [{'text': 'run it', 'type': 'text'}]}.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Prepared with Claude assistance; the bug was reproduced, and the change reviewed and tested, by me.