fix: treat example objects as values when rebuilding from describe()

[00200200]

Fixes #2480.

Joi.build() treated every object with a type property as a nested schema description. That is correct for keys, rules, and other schema terms, but examples are values. Rebuilding a schema whose example is { type: 'some-type' } (or constructing the same spec by hand) threw TypeError: this.joi[desc.type] is not a function.

This keeps example terms as literals during rebuild so describe() / build() round-trips, matching the existing { name: 'some-type' } case.

Testing

• Added a regression in example() plus a describe() round-trip in the manifest builder tests.
• Added native-type checks for Buffer, RegExp, and reference examples. Special descriptor decoding stays unchanged; the follow-up that treated reserved descriptor keys as plain objects was withdrawn.
• Confirmed the new tests fail on current master (this.joi[desc.type] is not a function) and pass with this change.
• Full suite: npm test — 1839 tests, 100% coverage, lint and types clean.

[jamalkamaladdin]

This check only guards desc.type with options.literal. A single value, ref, regex, buffer, function or override key still unwraps above this line, so build still misreads those examples.

[00200200]

Good point. Handled options.literal before the single-key descriptor checks so literal objects containing value, ref, regex, override, buffer, or function are preserved as plain objects during rebuild. Added tests covering each of those cases.

[jamalkamaladdin]

Moving options.literal above the single-key checks makes build return Buffer, RegExp and ref examples as plain objects. Joi.build(Joi.binary().example(Buffer.from('abc')).describe()) gives an object example, where master gives a Buffer.

[00200200]

Confirmed and fixed in c0ef7e4. Special single-key descriptors are decoded before checking the literal flag again. I withdrew the follow-up that treated reserved descriptor keys as plain objects, keeping this PR scoped to examples containing type. New tests verify actual Buffer, RegExp and reference types, not just describe() equality. Full npm test: 1839 tests, 100% coverage, lint and types clean.