fix(resource): serialize only writable fields in update payloads

[brd6]

Description

Fixes retrieve-then-update, which never worked against the live API on any version: update payloads carried every hydrated field, and the API rejects several of them.

• toArrayForUpdate() on Page, Database, and DataSource is now strict on the accepted update keys, instead of force-including them alongside every other non-empty field. This stops read-only top-level metadata leaking into bodies — created_time (which serialized as a date object and always failed validation), created_by, last_edited_*, url, parent — and subsumes the earlier object/id and null icon/cover special-casing.
• Page payloads additionally drop computed property values the API refuses (created_by, created_time, last_edited_by, last_edited_time, formula, rollup, unique_id — verification stays, being writable on wiki pages) and properties hydrated with a null value, whose serialized form carries no usable value ("date": null became "date": []). The filter resolves property names through the same snake_case key transform serialization applies, so camelCase-named properties are covered.
• Explicitly set values still serialize, including false (archived, is_locked) and 0; the archived: false round-trip behavior from Page object serializes archived: false on create and update, causing Notion API validation error #67 is preserved and still pinned.

Motivation and context

The natural "retrieve, mutate, update" flow failed with confusing validation errors on any real-world page (anything with computed or empty properties) — for example body.properties.Last edited by.title should be defined or body.created_time should be a string.

How has this been tested?

• New tests: computed property values dropped from update and create payloads (asserted under the transformed key names so they cannot pass vacuously), null-valued properties dropped, writable properties kept; the retrieve-then-update payload-shape test updated to the strict contract with its archived: false assertion intact.
• Verified live against the Notion API: retrieve-then-update now succeeds for a page (on both 2022-06-28 and 2026-03-11), a database row with a full property set, and a database — all of which failed before the change.
• An adversarial review pass caught the property-name transform mismatch, fixed and re-verified.
• Full suite green (218 tests), plus parallel-lint, phpcs, PHPStan, and Psalm.

Known remaining edge (separate issue): a data source schema round-trip still fails on computed property configurations, and camelCase-named properties are renamed by the payload key transform — both pre-existing and tracked separately.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.