[IO-780] Do not discard CharsetEncoder errors in ReaderInputStream

[tanvir-ux]

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Fixes IO-780.

ReaderInputStream.fillBuffer() called CharsetEncoder.flush() whenever the reader had reached EOF, overwriting lastCoderResult from encode(). When encode() reported malformed input (for example a trailing unpaired surrogate with the default CodingErrorAction.REPORT), the error was discarded and read() returned -1 instead of throwing.

This checks lastCoderResult.isError() after encode() and after flush().

The existing testCodingError() WIP now asserts CharacterCodingException for both read() and read(byte[]), matching the JIRA reproducer.

[garydgregory]

@tanvir-ux
Please follow the instructions in the PR template to avoid wasting time and resources on failing builds.

[tanvir-ux]

Sorry about the wasted CI — leftover unused assertDoesNotThrow import after flipping testCodingError to assertThrows. Removed it in 6df255f. ReaderInputStreamTest is 27/0 locally.

[garydgregory]

@tanvir-ux Thank you for the update, merged 🚀

[tanvir-ux]

Awesome! Thanks a lot!!