Preserve original multiline text in PSReadLine reverse-history insertion

[Copilot]

Reverse-history selection via Ctrl+r was rewriting multiline entries by appending continuation backticks to each non-final line, altering command semantics. This updates insertion to preserve the original history text exactly for multiline commands.

• Root-cause fix: multiline join delimiter

  • Updated Invoke-FzfPsReadlineHandlerHistory to join picked history lines with a plain newline instead of backtick-newline.
  • Effect: multiline entries are inserted as complete statements unless backticks were already present in the history line content.

• Behavior coverage for multiline history

  • Added focused tests for:
    • multiline complete statements (no synthetic backticks inserted)
    • multiline entries that already contain explicit continuation backticks (preserved as-is)

# before
$result = $result -join "```n"

# after
$result = $result -join "`n"