[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.