Modernize CI workflows and enforce strict linting

[e-desouza]

High Level Overview of Change

• Replace deprecated actions-rs/* with dtolnay/rust-toolchain and direct cargo commands (actions-rs has been archived)
• Upgrade actions/checkout from v1/v2 to v4
• Add cargo fmt --check step to quality workflow
• Enforce -D warnings on clippy in CI and pre-commit hook
• Add rustfmt.toml with import ordering rules
• Change pre-commit hook to check formatting rather than auto-fix
• Fix duplicate "Build only core" step name (was building json-rpc)

Context of Change

The CI workflows were relying on actions-rs/toolchain@v1, actions-rs/cargo@v1, actions-rs/clippy-check@v1, and actions-rs/audit-check@v1, all of which have been archived and are no longer maintained. GitHub has been printing deprecation warnings on every run.

The replacement approach uses dtolnay/rust-toolchain@stable (actively maintained, widely adopted) and runs cargo commands directly, which is the recommended pattern going forward.

The pre-commit hook was running cargo fmt (silently auto-fixing) and cargo clippy --fix --allow-staged (also silently fixing). This hides issues from developers. Changed to --check mode so problems are surfaced rather than papered over.

Depends on #128 (fix/clippy-and-formatting) since the codebase needs to be clean before strict enforcement is turned on.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Before / After

Before:

• CI uses archived actions-rs/* actions with deprecation warnings
• actions/checkout@v1 and @v2
• No cargo fmt check in CI
• Clippy runs without -D warnings (warnings are informational only)
• Pre-commit hook silently auto-fixes formatting and clippy issues

After:

• CI uses dtolnay/rust-toolchain@stable and direct cargo invocations
• actions/checkout@v4
• cargo fmt -- --check runs in the quality workflow
• cargo clippy --all-features -- -D warnings enforced in CI
• Pre-commit hook fails on violations instead of silently fixing them
• rustfmt.toml ensures consistent import grouping and ordering

Test Plan

Verified locally that all CI-equivalent commands pass:

cargo fmt -- --check
cargo clippy --all-features -- -D warnings
cargo test --all-features
cargo audit

The workflow YAML was validated for correct syntax. Actual CI execution will be verified when this PR triggers the workflows.