Bump **CI** Clippy and rustfmt toolchain to Rust 1.96

[spital]

Split out from the discussion at #1589 (comment)
per IvanIsCoding's suggestion to keep toolchain bumps separate from that PR.

The CI build/lint job pinned the Clippy/rustfmt toolchain at Rust 1.92
(# TODO: unpin clippy). Running

cargo clippy --workspace --all-targets --all-features -- -D warnings

with newer toolchains surfaced lints that newer Clippy enforces. This PR bumps
the pin to 1.96 (current stable) and fixes those lints.

Changes

• .github/workflows/main.yml — bump the build/lint dtolnay/rust-toolchain
  pin from 1.92 to 1.96 (drops the stale # TODO: unpin clippy). Clippy stays
  pinned on purpose, so CI doesn't break unexpectedly when a new Rust release
  adds lints; this just moves the pin forward to current stable.
• rustworkx-core/src/planar/lr_planar.rs — collapse the nested if inside
  the DfsEvent::BackEdge arm into a match guard. Fixes clippy::collapsible_match
  (reported by Rust 1.95). Behavior-neutral.
• rustworkx-core/src/{coloring,bipartite_coloring}.rs — drop a redundant &
  in three test-only panic!("{:?}", &edge.id()) calls. Fixes
  clippy::useless_borrows_in_formatting (reported by nightly Clippy).

Validation

Reproduced the failures and confirmed the fixes across toolchains:

• cargo +1.95 clippy --workspace --all-targets --all-features -- -D warnings
  (originally failed on collapsible_match; now passes)
• cargo +1.96 clippy --workspace --all-targets --all-features -- -D warnings
• cargo +nightly clippy --workspace --all-targets --all-features -- -D warnings
  (originally failed on useless_borrows_in_formatting; now passes)
• cargo +1.96 clippy --workspace --all-targets -- -D warnings (matches CI)
• cargo +1.96 fmt --all -- --check and cargo +nightly fmt --all -- --check
• cargo +1.96 test -p rustworkx-core (all pass, incl. planar suite)

[IvanIsCoding]

Thank you

[coveralls]

Coverage Report for CI Build 26846532752

Coverage decreased (-0.02%) to 94.711%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 4 of 4 lines across 1 file are fully covered (100%).
• 3 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

3 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
rustworkx-core/src/generators/random_graph.rs	3	86.73%

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Coverage Stats

Relevant Lines:	20213
Covered Lines:	19144
Line Coverage:	94.71%
Coverage Strength:	920199.24 hits per line

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💛 - Coveralls