fix(terminal-emulator): flush output stream after writing to PTY

[MaheshTechnicals]

Fixes pasted text being truncated by one or more bytes

When clipboard content is pasted and immediately followed by pressing Enter,
the last character(s) of the pasted text would sometimes be missing from
the command sent to the shell.

Root Cause:
The FileOutputStream used to write data to the PTY was never explicitly
flushed, allowing data to remain buffered in the output stream's internal
buffer. This created a race condition where:

1. Pasted text is written to the buffer but not sent to PTY
2. User presses Enter (sent immediately)
3. PTY receives incomplete pasted text + Enter keystroke

Example:

• Paste: 'sudo apt install git -y'
• After pressing Enter: shell receives 'sudo apt install git -'
• Result: command fails with 'Unknown option: -'

Solution:
Add explicit termOut.flush() call after each write operation in the I/O
writer thread to ensure data immediately reaches the PTY kernel buffer.

Changes:

• Added termOut.flush() after termOut.write() in TerminalSession.startThreads()
• Affects line 159 (now 160)
• Single line change with no side effects
• Improves all terminal I/O operations (paste, keyboard, mouse events)

Testing:

• Verified fix works for rapid paste operations
• Tested large paste operations (100KB+)
• Confirmed no performance impact (flush overhead is negligible)
• All I/O operations benefit from immediate delivery

Fixes: #paste-character-loss-bug