annotation: Warn instead of panicking on unparsable atom values

[tk1475]

An annotation with a value that doesn't parse, such as

/// cbindgen:rename-all=not-a-valid-rename-rule
#[repr(C)]
pub struct Trigger { pub field: u8 }

made cbindgen panic with called `Option::unwrap()` on a `None` value, because AnnotationSet::parse_atom did y.parse::<T>().ok().unwrap().

With this change parse_atom logs a warning with the existing warn! macro and returns None, so the annotation is ignored and the callers fall back to the configured default (for example structure.rename_fields for rename-all), the same as when the annotation is absent:

WARN: Couldn't parse value "not-a-valid-rename-rule" of annotation "cbindgen:rename-all", ignoring it.

This covers every parse_atom user (rename-all, rename-variant-name-fields, rename-associated-constant). An empty value (cbindgen:rename-all=) still maps to T::default() as before.

Fixes the rename-all panic from #1184. The other reproducers in that issue (generic arity mismatches, malformed lock file, recursive #[path] module) are separate and not touched here, so the issue should stay open.

How tested

• Added unit tests parse_atom_valid and parse_atom_invalid in src/bindgen/ir/annotation.rs. parse_atom_invalid panicked before the fix and passes after.
• Ran the issue's reproducer with cbindgen --lang c: it panicked before, and now prints the warning and generates the header.
• CBINDGEN_TEST_VERIFY=1 cargo +nightly test: unit tests, profile and all 161 tests/rust cases pass with no expectation changes. I don't have Cython or CMake locally, so I ran the expectation suite with CBINDGEN_TEST_NO_COMPILE=1, and the depfile tests (which need CMake) could not run.
• cargo +stable fmt --check and cargo +stable clippy --workspace -- -D warnings: clean.

AI disclosure

This change was drafted with Claude Code, then reviewed and tested by me.

🤖 Generated with Claude Code