Fix out-of-bounds read in eql_case_insensitive_ascii when the input is longer than the comparand (#31264)

### What does this PR do?

Fixes an ASAN `global-buffer-overflow` found by fuzzing the CSS parser:

```
asan:global-buffer-overflow:strncasecmp|eql_case_insensitive_ascii|eql_case_insensitive_ascii|bun_core::string::immutable::eql_case_insensitive_ascii_ignore_length
```

**Repro**

```sh
BUN_FEATURE_FLAG_INTERNAL_FOR_TESTING=1 bun -e 'require("bun:internal-for-testing").cssInternals.minifyTest(":nth-child(Nn", "")'
```

```
==ERROR: AddressSanitizer: global-buffer-overflow READ of size 2 ...
    #0 strncasecmp
    #1 bun_core::strings_impl::eql_case_insensitive_ascii src/bun_core/lib.rs
    #2 bun_core::string::immutable::eql_case_insensitive_ascii_ignore_length src/bun_core/string/immutable.rs
    #3 bun_css::css_parser::nth::parse_nth src/css/css_parser.rs
    #4 bun_css::selectors::parser::parse_nth_pseudo_class src/css/selectors/parser.rs
```

**Cause**

`strings_impl::eql_case_insensitive_ascii(a, b, check_len)` defers to
`strncasecmp(a, b, a.len())`, which reads up to `a.len()` bytes from
*both* buffers. The Zig original (`strings.eqlCaseInsensitiveASCII`)
compared against NUL-terminated comptime literals, so `strncasecmp`
stopped at the sentinel and reported a mismatch whenever `a` was longer
than `b`. Rust byte-string literals carry no terminator, so the An+B
parser's ident branch (`parse_nth`), which compares an arbitrary user
ident against the keywords `"even" / "odd" / "n" / "-n" / "n-" / "-n-"`
with the ignore-length variant, reads past the end of the keyword
literal as soon as the ident is longer than the keyword and shares its
prefix (`Nn` vs `n`, `n-3` vs `n`, …). Besides the OOB read, the
comparison result depended on whatever byte happens to follow the
literal in rodata.

**Fix**

Reject `b.len() < a.len()` up front in `eql_case_insensitive_ascii`
before calling `strncasecmp` — the same result the NUL sentinel produced
in Zig, so observable behavior is unchanged for every in-bounds input
(all other callers of the ignore-length variant already pass
equal-length slices). `strncasecmp` now only ever reads within both
slices.

**Verification**

- `bun bd test test/js/bun/css/nth-anplusb-ident.test.ts` without the
fix (src/ stashed): aborts with the ASAN global-buffer-overflow above.
- With the fix: passes. The new test covers valid `n-<digits>` idents
that are longer than the `n`/`n-` keywords (`:nth-child(n-3)`,
`:nth-child(N-3)`, `:nth-last-child(n- 42)`), keyword case-insensitivity
(`:nth-child(N)`), an invalid ident (`:nth-child(NN)` → parse error),
and the exact fuzzer-minimized input run in a subprocess.
- `bun bd test test/js/bun/css/css.test.ts`: 1032 pass, 0 fail (no
behavior change for the existing suite).
- A second fuzz report hits the same overflow through `Bun.build` with a
CSS entrypoint containing `:nth-child(Nn`; that path goes through the
same `parse_nth` comparison and is covered by this fix (`Bun.build` now
reports a parse error instead of aborting).
- The `build-rust` CI failures on this PR (unused label / unnecessary
`unsafe` warnings in `src/spawn`, `src/install`, `src/crash_handler`,
`src/runtime/ffi`, `src/runtime/dns_jsc`) are present on current `main`
commits that don't include this change and come from files this PR
doesn't touch.