feat(core): add a pluggable Form Validator next to Ajv

[gigibiffi84]

Part of #1498. Follows the agreement to land validator decoupling incrementally and non-breaking in 3.x, shaped after the FormValidator / ValidationIssue types on feat/presentation-model (#2571).

What changed

@jsonforms/core gains a pluggable validation seam next to Ajv:

• InitActionOptions.validator: a FormValidatorFactory (schema) => FormValidator, or a FormValidator already bound to the schema. Precedence: validator, then ajv, then the default Ajv instance.
• FormValidator has validate(data): ValidationIssue[] (synchronous in 3.x) and an optional matches(schema, data) for rule conditions (wired up in a follow-up PR).
• ValidationIssue mirrors 4.x (path, key, message, severity) plus optional params and parentSchema.
• The reducer creates the Form Validator through the factory whenever the schema changes and caches it in state.formValidator. state.validator keeps holding the compiled Ajv function whenever Ajv validates.
• issuesToErrors converts issues from custom validators to the Ajv error shape the rest of core and the renderers read: missing key becomes custom, a required issue addressed to the missing property is split into parent path plus params.missingProperty, non-error severities are dropped, and a missing parentSchema is resolved from the form schema by path. Ajv-backed validators bypass the conversion, so Ajv users get identical errors.
• Built-in adapter: createAjvValidator(ajv?); compiledAjvValidator(validateFn) wraps Ajv standalone code for CSP setups that keep Ajv but cannot allow unsafe-eval.
• New selector getValidator(state).

For adopters

Nothing changes without the new option: all existing core tests pass unchanged. One small fix: setValidationMode('NoValidation') now also drops the cached validator, as init and updateCore already did; previously a later data update could reintroduce errors in that mode.

Tests

• test/util/formValidator.test.ts: lossless Ajv round trip, every normalization rule, resolveParentSchema over properties, items, additionalProperties and $ref, both adapters, guards.
• test/reducers/core.test.ts: a fake validator receives every compile and validation, Ajv is never asked to compile when a validator is given, bound validators, updateCore recompiling only on schema change, options without a validator key keeping the current validator, explicit undefined switching back to Ajv, update, setSchema, NoValidation, setAjv precedence, issue normalization, getValidator.
• 466 core tests green; @jsonforms/react, @jsonforms/vue and @jsonforms/angular build against the new types; typedoc succeeds.

Follow-ups (separate PRs)

Rule conditions through matches / the factory; the validator prop in the React, Angular and Vue bindings with an example; a Structural Matcher fallback for rules.

🤖 Generated with Claude Code

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