fix(tool): align JSON contract with conditional fetch fields

[chaliy]

Motivation

• Typed FetchRequest and FetchResponse gained conditional-fetch fields (if_none_match, if_modified_since, etag) but the tool JSON contract (argument validator and input/output schemas) was not updated, causing JSON/MCP callers to be rejected and tooling to fail strict schema validation.
• This PR fixes the API contract divergence so JSON callers and strict schema consumers see the same fields the typed Rust API uses.

Description

• Allow if_none_match and if_modified_since in the tool argument validator by updating validate_args in crates/fetchkit/src/tool.rs.
• Add if_none_match and if_modified_since to the input schema in build_input_schema with string types and descriptions so the tool definition advertises the fields.
• Add etag to the output schema in build_output_schema so serialized FetchResponse objects match the advertised schema.
• Add/extend unit tests to assert the input/output schemas expose the new fields and to ensure Tool::execution accepts conditional fetch arguments (test_execution_accepts_conditional_fetch_arguments, and updated test_tool_schemas).

Testing

• Ran cargo fmt --all which completed successfully.
• Ran targeted unit tests: cargo test -p fetchkit test_tool_schemas and cargo test -p fetchkit test_execution_accepts_conditional_fetch_arguments, both of which passed.
• Verified the updated tests in crates/fetchkit/src/tool.rs exercise schema exposure and argument validation and succeeded in CI-like local test runs.

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Codex Task