MOD-12261 add CI + fuzz tests

[AvivDavid23]

Note

Add new CI with build/test and smoke fuzzing via composite actions, introduce a fuzz crate/target, and refactor Cargo to a workspace with shared serde deps.

• CI:
  • Introduces ./.github/workflows/ci.yml with jobs: build_and_test and matrix fuzz_tests (fuzz_json_de), using concurrency control.
  • Removes legacy workflow ./.github/workflows/toolchain.yml (check/clippy/miri/coverage pipeline).
• GitHub Actions (composite):
  • Adds ./.github/actions/build_and_test to format, build, and run tests on Rust 1.88.
  • Adds ./.github/actions/fuzz_tests to install cargo-fuzz and run fuzz targets with a time limit.
• Fuzzing:
  • Adds fuzz/ crate with target fuzz_json_de generating arbitrary JSON and deserializing into ijson::IValue (uses libfuzzer-sys; nightly toolchain in fuzz/rust-toolchain.toml).
• Cargo/workspace:
  • Converts to a workspace adding member fuzz.
  • Moves serde/serde_json to [workspace.dependencies] and references them from the main crate.

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