feat(temporal-zod): support custom validation errors (#54)

[macalinao]

Closes #54.

Summary

Adds custom validation errors to temporal-zod via an .error() method on every validator — non-breaking, so existing code using the validators directly keeps working:

import { zPlainDate } from "temporal-zod";

// unchanged — default error
z.object({ date: zPlainDate });

// customized — takes Zod's `error` param (string or error-map fn)
z.object({ date: zPlainDate.error({ error: "Invalid date" }) });
z.object({ date: zPlainDate.error({ error: (issue) => `Bad: ${issue.input}` }) });

The custom error surfaces for any invalid input (malformed string or wrong type), works on both the coercing and instance validators, on the main entry and temporal-zod/base.

Implementation notes

• temporalValidators(cls, { extraInputs?, error? }) threads error into the z.union + z.instanceof — which is what Zod 4 actually surfaces (the union masks the inner transform message), so the default error is unchanged ("Invalid input").
• withError() augments a schema with a chainable .error() method via Object.assign. Verified safe: z.string().error is undefined (no collision), the augmented schema parses correctly in z.object/z.union, and z.toJSONSchema is unaffected.
• json-schemas.ts reuses each base validator's own .error() to rebuild, then re-applies JSON Schema metadata. The default keeps its registered $def; a custom-error variant inlines an identical schema (a distinct instance can't share the registered id — dedup is just an optimization).
• New public types: TemporalError, TemporalErrorParams, WithError, TemporalValidator, TemporalInstanceValidator.

Verification

• bun run build ✅ · bun run lint ✅ · bun test — 114 pass / 0 fail ✅
• Coverage added for string + function errors, nesting in z.object, unchanged default messages, and preserved JSON-Schema metadata. Existing tests use the validators directly (unchanged), confirming non-breaking.

Versioned as a minor bump with a changeset (new feature, non-breaking).

Note on PR #68

The in-flight oxlint/oxfmt migration (#68) also edits temporal-validator.ts. This branch is off current master (Biome/ESLint); whichever merges first, the other needs a trivial rebase. The new code is written to satisfy both lint setups.