fix(proxy): handle Read argument snapshot deltas

[SaladDay]

Summary

• Fix Responses streaming fallback for Read tool arguments when response.function_call_arguments.done omits arguments.
• Keep normal split JSON delta behavior by joining chunks, while supporting non-standard snapshot-style deltas by using the latest complete JSON object.
• Preserve the existing Read.pages == "" sanitizer and avoid rewriting/clamping offset because the proxy does not have file context.

Refs #197

Verification

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo test --manifest-path src-tauri/Cargo.toml read_tool_snapshot_argument_deltas_use_latest_complete_json
• cargo test --manifest-path src-tauri/Cargo.toml read_tool_split_argument_deltas_are_still_joined
• cargo test --manifest-path src-tauri/Cargo.toml read_tool_incomplete_argument_deltas_are_not_treated_as_snapshots
• cargo test --manifest-path src-tauri/Cargo.toml read_tool_duplicate_start_preserves_buffered_arguments
• cargo test --manifest-path src-tauri/Cargo.toml non_read_tool_empty_delta_still_uses_done_arguments
• cargo test --manifest-path src-tauri/Cargo.toml read_tool_arguments_are_buffered_and_sanitized
• git diff --check

[SaladDay]

cc @unive3sal

[unive3sal]

LGTM