Fixes#23275
### What does this PR do?
This PR fixes a bug where `bunfig.toml` files starting with a UTF-8 BOM
(byte order mark, `U+FEFF` or bytes `0xEF 0xBB 0xBF`) would fail to
parse with an "Unexpected" error.
The fix uses Bun's existing `File.toSource()` function with
`convert_bom: true` option when loading config files. This properly
detects and strips the BOM before parsing, matching the behavior of
other file readers in Bun (like the JavaScript lexer which treats
`0xFEFF` as whitespace).
**Changes:**
- Modified `src/cli/Arguments.zig` to use `bun.sys.File.toSource()` with
BOM conversion instead of manually reading the file
- Simplified the config loading code by removing intermediate file
handle and buffer logic
### How did you verify your code works?
Added comprehensive regression tests in
`test/regression/issue/23275.test.ts` that verify:
1. ✅ `bunfig.toml` with UTF-8 BOM parses correctly without errors
2. ✅ `bunfig.toml` without BOM still works (regression test)
3. ✅ `bunfig.toml` with BOM and actual config content parses the content
correctly
All three tests pass with the debug build:
```
3 pass
0 fail
11 expect() calls
Ran 3 tests across 1 file. [6.41s]
```
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Co-authored-by: Claude <noreply@anthropic.com>
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