Drop the Dart binding's unused serde_json dependency

[ifaouibadi]

Summary

• The codegen-sync fix on develop added serde_json to bindings/dart/rust/Cargo.toml on the assumption the generated SSE codec referenced it directly; the committed frb_generated.rs never does, so the dependency was dead weight.
• Confirmed by regenerating from a clean Cargo.toml: zero diff in the generated Dart/Rust glue, so serde_json was never load-bearing.

Test plan

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --release --locked
• flutter_rust_bridge_codegen generate --config-file flutter_rust_bridge.yaml then git status --porcelain clean (no drift)
• dart analyze --fatal-infos
• dart test (92/92 passing)

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