fix: coerce JSON-stringified dict/list tool params before Pydantic validation

[giulio-leone]

Issue

Closes #1285

Problem

When using strands with Bedrock/Claude and MCP tools that have nested object parameters (e.g., parameters: dict[str, Any] | None), the model incorrectly serializes the parameter value as a JSON string instead of a native Python dictionary:

# Expected (works with Gemini):
parameters={'ecs_cluster_name': 'my-cluster', 'time_window': 3600}

# Actual (Bedrock/Claude):
parameters='{"ecs_cluster_name": "my-cluster", "time_window": 3600}'

This causes a Pydantic validation error:

ValidationError: 1 validation error for call[...]
parameters
  Input should be a valid dictionary [type=dict_type, input_value='{...}', input_type=str]

Root Cause

The Bedrock Converse API returns tool_use.input as a parsed dict, but for nested object/array parameters the model may stringify the value before embedding it in the outer dict. The SDK's Pydantic-based validate_input() then receives a str where it expects a dict/list and rejects it.

Solution

Added a pre-validation coercion step in FunctionToolMetadata.validate_input() via _coerce_json_string_params() that:

1. Inspects each field's type annotation on the input model
2. If the annotation accepts dict or list (including Optional/Union/Any) and the incoming value is a string, attempts json.loads()
3. Only replaces the value if deserialization produces a dict or list
4. Leaves non-JSON strings untouched (they still fail Pydantic validation with a clear error)

Design Notes

• Model-provider-agnostic: The fix is at the tool decorator level, not Bedrock-specific, so it benefits any provider that exhibits this behavior
• Zero impact on correct inputs: The coercion only activates when a string value is received for a dict/list field; dict/list values pass through unchanged
• Backward compatible: No changes to any public API or schema generation
• Conservative: Only coerces to dict or list — won't coerce a string that json.loads to an int, bool, etc.

Testing

• Added test_validate_input_coerces_json_string_to_dict: Tests dict coercion with Optional[dict[str, Any]], verifies dict values still work, None still works, and non-JSON strings still raise ValueError
• Added test_validate_input_coerces_json_string_to_list: Tests list coercion with list[str]
• All 82 decorator tests pass (80 existing + 2 new)
• All 439 tools module tests pass

Changes

• src/strands/tools/decorator.py: Added _coerce_json_string_params(), _annotation_accepts_mapping_or_sequence() methods to FunctionToolMetadata; updated validate_input() to call coercion before Pydantic validation
• tests/strands/tools/test_decorator.py: Added 2 test cases for JSON string coercion

[giulio-leone]

Friendly ping — coerces JSON-stringified dict/list tool parameters back to native types before Pydantic validation, fixing crashes when LLMs return string-wrapped JSON.