Let UseEnum select a member by value, not only by name or number

[sap1110]

Closes #827

UseEnum resolves a member from its name, its scoped name, or (for int-valued enums) its number. There is no lookup by value, so a str-based enum whose member names differ from their values cannot be set by value:

class Color(str, enum.Enum):
    RED = "red"
    GREEN = "green"
    BLUE = "blue"

class MyEntity(HasTraits):
    color = UseEnum(Color, default_value=Color.BLUE)

MyEntity().color = "GREEN"   # fine, that is the member name
MyEntity().color = "green"   # TraitError, though it is Color.GREEN's value

TraitError: The 'color' trait of a MyEntity instance expected any of
['RED', 'BLUE', 'GREEN'], not the str 'green'.

The docstring example hides this because its member names are lowercase and equal to nothing else, so "green" happens to match a name. Give the members conventional uppercase names and the string form stops working entirely.

select_by_value is the non-numeric counterpart of the existing select_by_number, and the string branch of validate now falls back to it.

The change is additive. Name lookup is still attempted first, so every string that resolved before resolves to the same member; the new lookup only runs where a TraitError was previously raised. There is a test using an enum whose member name shadows a different member's value, so the precedence is pinned rather than incidental.

One thing worth a second opinion: info() still lists only member names, so the error text for a genuinely bad value reads expected any of ['RED', 'BLUE', 'GREEN'] even though values are now accepted too. Widening it would change message text that downstream suites may match on, so I left it alone. Happy to include it if you'd rather they agree.

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🤖🍆 Prepared with the help of an AI coding agent, marked as the contributing guide asks agents to do. Reviewed and tested by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.17%. Comparing base (80756bf) to head (971a590).

Components	Coverage Δ
traitlets	85.53% <100.00%> (+0.03%)	⬆️
tests	99.07% <100.00%> (+<0.01%)	⬆️

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