feat(jsonschemagen): emit propertyNames from inlined-dict key slot constraints

[jdsika]

Summary

For an inlined-as-dict slot (inlined: true, inlined_as_list: false) whose range
class has an identifier/key slot, render the key slot's string-applicable
constraints onto JSON Schema propertyNames. Previously these were silently
dropped, leaving the generated schema strictly looser than the model declares.

Motivation (standards)

Grounded at both ends:

• LinkML inlining semantics (https://linkml.io/linkml/schemas/inlining.html) — in
  the inlined-dict form the dictionary key is the identifier value of the range
  class (not repeated in the value object: "it is acting as a key for that object, it
  doesn't need to be included"). So constraints on the identifier/key slot are, by
  LinkML's serialization, constraints on the keys.
• JSON Schema Core, draft 2019-09, §9.3.2.5 (propertyNames, Applicator
  vocabulary) — the subschema is applied to each property name, which "will always
  be a string." propertyNames composes conjunctively with the existing
  additionalProperties value schema.

Changes

• Derive the key constraints from the key slot via the existing
  get_value_constraints_for_slot helper, then emit the string-applicable subset
  onto propertyNames: pattern, enum (equals_string_in), and a string
  const (equals_string).
• Numeric constraints are intentionally excluded because property names are always
  strings: minimum/maximum are no-ops on strings, and a numeric const
  (equals_number) would reject every key. allOf (from range_expression) is
  likewise not emitted onto keys.
• structured_pattern is honored when materialize_patterns is enabled, identical to
  how value patterns are handled (documented in-code).
• Backward compatible: propertyNames is emitted only when a string-applicable key
  constraint applies.

Notes

• propertyNames is available since draft-06; the generator emits draft 2019-09.
• Regex dialect (ECMA-262) is consistent with the generator's existing pattern
  emission.

How was this tested?

Parametrized over identifier/key key slots; covers literal pattern,
equals_string_in (enum), string equals_string (const), structured_pattern
(with/without materialization), and behavioral validate good/bad-key checks. Negative
controls assert that a numeric const and numeric minimum/maximum produce no
propertyNames (so numeric-string keys are not wrongly rejected), and that an
unconstrained key slot is unchanged.

Areas of uncertainty

• propertyNames is emitted only from the key slot's string-applicable constraints (see Notes); non-string key ranges and interactions with additionalProperties follow JSON Schema draft-07 semantics and are covered by tests, but other drafts were not evaluated.

Checklist

• My code follows the contributor guidelines
• I have added tests that prove my fix/feature works
• Existing tests pass locally with my changes

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