fix(models): preserve string content in anthropic tool results

[lawrence3699]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: AnthropicLlm.part_to_message_block() iterates "content" string char-by-character when LoadSkillResourceTool is used with Claude #5358

Problem:
part_to_message_block() currently assumes every function_response.response["content"] value is a list-like content payload. LoadSkillResourceTool returns a plain string in content, so Claude tool results get flattened character-by-character with newlines inserted.

On current main, this deterministic repro returns the wrong content:

from google.adk.models.anthropic_llm import part_to_message_block
from google.genai import types

part = types.Part.from_function_response(
    name="load_skill_resource",
    response={
        "skill_name": "my-skill",
        "file_path": "references/doc.md",
        "content": "Hello",
    },
)
print(repr(part_to_message_block(part)["content"]))
# 'H\\ne\\nl\\nl\\no'

Solution:
Keep the existing list handling for Anthropic content blocks, but branch on the runtime type of response["content"]:

• list: preserve the current block flattening behavior
• dict: JSON-serialize it
• scalar/string (including ""): pass it through as a scalar string

That preserves LoadSkillResourceTool string payloads without changing the existing content list contract.

Testing Plan

Unit Tests:

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

Passed pytest results:

• pytest tests/unittests/models/test_anthropic_llm.py -q
  • 43 passed, 1 warning in 1.38s
• pytest tests/unittests/tools/test_skill_toolset.py -k 'load_resource_run_async' -q
  • 10 passed, 71 deselected, 2 warnings in 1.27s

Manual End-to-End (E2E) Tests:

Minimal local repro after the patch:

from google.adk.models.anthropic_llm import part_to_message_block
from google.genai import types

for value in ["Hello", ""]:
    part = types.Part.from_function_response(
        name="load_skill_resource",
        response={
            "skill_name": "my-skill",
            "file_path": "references/doc.md",
            "content": value,
        },
    )
    print(repr(value), '=>', repr(part_to_message_block(part)["content"]))

Console output:

'Hello' => 'Hello'
'' => ''

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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

I also checked upstream overlap before opening this PR:

• #5364 already has open PR #5366, so I did not touch that nearby schema bug.
• I found no open PR for #5358 or another in-flight part_to_message_block() string-content fix.

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