Exclude byte-order mark from ExtendedBufferedReader byte counting

[rootvector2]

getEncodedCharLength re-encodes each character with the configured CharsetEncoder to count its bytes, but the convenience encode call re-emits a byte-order mark on every invocation for charsets like UTF-16. Parsing a UTF-16 stream with setTrackBytes(true), for example an Excel Unicode export, counts every code unit as 4 bytes instead of 2, so CSVRecord.getBytePosition() grows at twice the true rate. Found by comparing reported byte offsets against the encoded prefix length across charsets.

Measure the per-call BOM once and subtract it from each encoded length. The count already lives in getEncodedCharLength, so keeping the adjustment there covers both the single-char and array read paths, and it stays a no-op for charsets that emit no BOM (UTF-8, UTF-16LE/BE, single-byte).

[Copilot]

Pull request overview

This PR fixes byte-position tracking when parsing CSV data with trackBytes=true and a BOM-emitting charset encoder (notably UTF-16), preventing byte offsets from being inflated by a per-encode-call BOM.

Changes:

• Compute and cache the per-call BOM length emitted by CharsetEncoder.encode(...) and subtract it from per-character encoded byte counts.
• Add a regression test ensuring CSVRecord.getBytePosition() advances at the correct rate for UTF-16 when byte tracking is enabled.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/main/java/org/apache/commons/csv/ExtendedBufferedReader.java	Measures per-call BOM emission once and subtracts it from byte-count calculations to keep bytesRead accurate for BOM-emitting encoders.
src/test/java/org/apache/commons/csv/CSVParserTest.java	Adds coverage for correct byte-position advancement under UTF-16 with byte tracking enabled.

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[garydgregory]

Thank you @rootvector2 , merged 🚀
Would you please add a similar test for UTF-32 BE/LE?