Fix history saving when directory writability is unreliable

[st0012]

Problem

• Issue #1219 reports that IRB on Windows can warn Can't write history ... due to insufficient permissions on exit even though writing the same history file manually succeeds.
• The failing preflight came from IRB::HistorySavingAbility#ensure_history_file_writable, which checked history_file.dirname.writable? before attempting the history write.
• On Windows, Ruby's w32_access checks synthetic Unix-style mode bits derived from Windows file attributes; that path can disagree with the actual write operation for directories.
• Microsoft documents that the read-only attribute is not honored on directories, so a directory-level writable predicate is the wrong source of truth for whether IRB can create or update a history file.

Solution

• This changes history saving to normalize permissions only for an existing history file, then attempt the real File.open write and report ENOENT, EACCES, or EPERM from that operation.
• This keeps the existing owner-only permission normalization behavior for existing history files while removing the parent directory writable? gate.
• This adds a Windows-only regression test that sets a temporary history directory read-only with attrib +R, verifies that a normal file write still succeeds, runs an IRB session, and asserts that the history file is saved without warnings in test_history_windows.rb.
• This shares the history test fixture through HistoryTestCase, so HistoryTest and HistoryWindowsTest use the same setup without making the Windows regression inherit the full non-Windows test suite.
• The new file is included by the repository's test/irb/**/test_*.rb test pattern, while its own windows? guard omits the case on non-Windows platforms.