Surface orphan bits in @export_flags inspector after a flag is removed

[basteez]

Bugfix

Fixes #72010

Problem

When a flag is removed from an @export_flags declaration, any bit that was set for the removed flag stays in the stored value. EditorPropertyFlags only builds checkboxes for flags that still exist, so:

• update_property() only reflects bits that have a checkbox, so the orphaned bit never shows up in the inspector;
• _flag_toggled() reads the full stored value and only OR/ANDs the toggled bit, so the orphaned bit survives every toggle.

The result is a bit that can't be cleared from the inspector and keeps being reported at runtime, e.g. the reproduction from the issue still printing 4 after "Test 3" is removed.

Change

• Accumulate all_flags_mask (the OR of every value that maps to a current flag) while building the checkboxes in setup().
• In update_property(), surface every stored bit that maps to no current flag as an extra "Bit N (unknown flag)" checkbox, wired to the same _flag_toggled() path as regular flags. The user can untick it to clear the orphaned bit, and nothing is silently dropped.
• When every named flag has been removed, the orphan checkboxes are the only controls, so the property's label/focus reference is anchored to the first one.

Values with no orphaned bits are unaffected — identical UI and identical emitted values.

Alternative considered

A smaller alternative is to simply mask out orphaned bits and emit the cleaned value on load. It was rejected here because it rewrites stored data on open (marking scenes/resources modified and dropping bits without the user's knowledge). Happy to switch to that approach if maintainers prefer the minimal patch.

How to verify

1. Attach a script with @export_flags("Test", "Test 2", "Test 3") var test_flags = 0 and print test_flags in _ready().
2. Enable the last flag, then remove "Test 3" from the declaration.
3. Before this change the removed flag's bit is invisible and still prints; with this change it appears as a "Bit 2 (unknown flag)" checkbox that can be unticked to clear it.

Known limitation (pre-existing, out of scope)

Partially-set multi-bit flags declared with an explicit value (e.g. "Combo:6") can still leave an individual bit without UI representation. This is pre-existing behavior unrelated to the positional-flag case in #72010 and would be better addressed separately.