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Fix panic on BOM-prefixed files in diagnostic renderer - #30017
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fixes #29958
Summary
When rendering diagnostics for a file with UTF-8 BOM encoding, the diagnostics renderer removes the 3 bytes of BOM from the snippet buffer, yet the annotations that started from byte 0 would underflow and fall back to their uncorrected offset, which exceeded the new length of the buffer and caused panic. The changes ensure the bounds are adjusted using saturating subtract, such that they stay within snippet bounds.
Test Plan
Added unit tests covering BOM-prefixed files and tested the minimal reproduction from the issue locally.