fix: carry Anthropic tool input schemas as-is

[CodePrometheus]

Description

An Anthropic tool declares its input as a JSON schema, but the gateway modeled that schema as a struct with only "type", "properties" and "required".
Everything else a client sends is silently dropped on the way to the backend: "additionalProperties", which backends need to enable strict schema adherence, the per-property "description" the model relies on to call the tool correctly, "$defs", and so on.
We hit this with a tool that takes no arguments: its empty "properties" was dropped too, and the OpenAI-compatible backend rejected the whole request with 400 invalid_function_parameters, "object schema missing properties".

The schema is now carried as raw JSON, so tools reach the backend as the client declared them. This covers both the Anthropic-to-OpenAI and the Anthropic-to-Bedrock conversions, which shared the struct.

Related Issues/PRs (if applicable)

Special notes for reviewers (if applicable)

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.05%. Comparing base (3fc0f4f) to head (18f8662).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2472   +/-   ##
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  Coverage   85.05%   85.05%           
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  Files         159      159           
  Lines       22649    22649           
=======================================
  Hits        19265    19265           
  Misses       2217     2217           
  Partials     1167     1167           

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