FAC-2305 Hydrate response dates consistently in SDK 6.0

[javorosas]

Summary

• Hydrate ISO date fields across SDK JSON responses and locally validated webhook events.
• Align public response date types and nullable invoice, retention, and customer dates with runtime values.
• Preserve SAT stamp.date as a wall-clock string and leave caller metadata untouched.

Release coordination

This PR contains only the date/type correction and its tests. The SDK 6.0 version bump, changelog, dependency updates, ESLint configuration, and ESM/CommonJS packaging are in a separate PR based on this branch. Merge this PR first, then the release PR; these changes require a major release.

Validation

• pnpm install --frozen-lockfile
• pnpm test: build, 56 runtime tests, type tests
• pnpm run lint

[Copilot]

Copilot review overview

🔵 Needs a closer look

Add runtime coverage confirming cancellation.last_checked hydrates to a Date.

Review effort: Lite
Findings: None

What changed in this PR

Updates SDK 6.0 to hydrate ISO response dates as Date objects while preserving SAT wall-clock timestamps and caller metadata.

Changes:

• Added centralized date hydration for API responses and validated webhooks.
• Updated nullable and newly exposed date types.
• Added runtime/type tests and documented the breaking release.

File	Description
test/​web/​runtime-compat.web.test.ts	Tests browser date hydration.
test/​node/​signed-download-urls.node.test.ts	Tests signed URL dates.
test/​node/​runtime-compat.node.test.ts	Tests Node response and webhook hydration.
test-d/​runtime-types.test-d.ts	Verifies updated TypeScript types.
src/​wrapper.ts	Adds centralized response date hydration; coverage is needed for cancellation.last_checked.
src/​types/​retention.ts	Aligns retention date types.
src/​types/​organization.ts	Aligns organization date types.
src/​types/​invoice.ts	Updates nullable and cancellation date types.
src/​types/​common.ts	Updates shared date types.
src/​tools/​webhooks.ts	Hydrates locally validated webhook events.
package.json	Bumps version to 6.0.0.
CHANGELOG.md	Documents breaking changes.

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[javorosas]

Atendí la observación general de Copilot en d8a90a4: la prueba runtime existente ahora verifica explícitamente que cancellation.last_checked se hidrate como Date. Verificado con pnpm run test:node (46 pruebas) y pnpm run lint. Copilot no creó review threads inline que requieran resolveReviewThread.

[javorosas]

Consolidated into #121, which now targets main and includes all changes from this branch. Continue review and release coordination in #121.