Defer EntryPoint value validation until loading

[rudra337-lgtm]

Fixes #523.

EntryPoint.__init__ previously parsed and validated value immediately. Since entry_points(group=...) constructs entries before applying the group filter, one malformed entry in an unrelated group could prevent callers from retrieving valid entries. Defer parsing until a component is accessed or load() is called; malformed entries still raise ValueError when used.

The regression test creates a distribution with a valid selected group and an invalid unselected group, verifies selection succeeds, and verifies loading the invalid entry still raises ValueError.

Validation:

• tests/test_api.py: 38 passed, 39 subtests passed.
• Existing TestEntryPoints unit tests: 8 passed.
• Ruff and git diff --check passed.

AI assistance was used to prepare and validate this change; the reported checks were run locally.