Add getOutputMessage() accessor to Statement

[fdcastel]

Based on #44

Adds a getOutputMessage() accessor to the Statement class, addressing REQ-2 from #42.

Problem

Statement exposes getInputMessage() for raw write access to the input parameter buffer, but there was no corresponding getOutputMessage() for the output result buffer (outMessage was private).

The ODBC driver needs raw access to the output message buffer for:

1. Prefetch optimization: Copying raw fetched rows into a prefetch buffer after each fetchNext().
2. Direct descriptor integration: Pointing OdbcConvert descriptor pointers at offsets within the output message, eliminating a separate buffer and one memcpy per row.

Changes

• Statement: Added getOutputMessage() returning std::vector<std::byte>&, symmetrical with getInputMessage().
• Tests: Added test verifying the buffer is non-empty after prepare and that the reference remains stable across accesses.

Impact

• Backward compatible: No existing APIs or behavior changed.
• Enables the ODBC driver to eliminate the separate Sqlda.buffer for output data and implement N-row prefetch using fb-cpp's buffer as the source.

[asfernandes]

Please rebase.

[fdcastel]

Rebased onto latest main.