[Flight] Preserve leading `U+FEFF` in text rows (#37625)

The Flight client dropped a leading `U+FEFF` from outlined text rows
when it decoded binary input. `TextDecoder` consumed the character as an
encoding signature, even though it was part of the serialized string.
Both the Node and Web decoders now use `ignoreBOM: true` to preserve it.

Regression tests cover one and two leading U+FEFF characters with normal
chunks and with every UTF-8 byte delivered separately. They also include
an inline-string control. The global `TextDecoder` Flow declaration now
makes `fatal` optional and declares `ignoreBOM`, matching the API.