[rust-compiler] Fix non-ASCII panic in ValidateNoSetStateInEffects (#37539)

## Summary

`get_identifier_name_with_loc` reads an identifier's name out of the
source when SSA has dropped it. `SourceLocation.index` is a Babel
position and counts UTF-16 code units, but the fallback used it to slice
a Rust `&str`, which indexes UTF-8 bytes:

```rust
let slice = &code[start_idx..end_idx];
```

These agree only while the source is ASCII. After any non-ASCII
character, later offsets are short by the extra bytes, so the slice
reads the wrong span, or panics when it lands inside a character:

```
panicked at crates/react_compiler_validation/src/validate_no_set_state_in_effects.rs:168:30:
start byte index 637 is not a char boundary; it is inside 'う' (bytes 636..639 of string)
```

The panic aborts validation for the whole file, so every diagnostic in
it is lost.

`validate_no_derived_computations_in_effects.rs` already converts
correctly in its own copy of this function. Both were added in #36173
and only one got the UTF-16 handling, so this ports that logic over.

Found through Biome, which embeds these crates for its
`useReactCompiler` rule.

## How did you test this change?

Added `repro-setState-in-effect-non-ascii-source.ts`, which panics
before this change and compiles cleanly after:

```
bash scripts/test-rust-port.sh ValidateNoSetStateInEffects \
  packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-setState-in-effect-non-ascii-source.ts
```

The non-ASCII comments in it are test input, not documentation; removing
them realigns the offsets and the crash disappears.

Everything `compiler_rust.yml` runs is green on macOS arm64, including
`scripts/test-rust-port.sh` (1811 passed) and `yarn snap --rust` (1812
passed).