[Feature] Add filter field support to webhook handlers

[vsmityukh]

[Feature] Add filter field support to webhook handlers

WHY are these changes introduced?

Shopify's GraphQL Admin API supports a filter parameter on webhook
subscriptions (WebhookSubscriptionInput.filter) that limits which
payloads get delivered based on a filter expression (e.g. "title:shoes").
The library already supports analogous fields — includeFields and
metafieldNamespaces — but filter was never wired up, making it
impossible to use this API capability through the SDK.

WHAT is this pull request doing?

Adds filter?: string support to webhook handlers, following the exact
same pattern as includeFields and metafieldNamespaces:

• types.ts: Added filter?: string to BaseWebhookHandler (inherited
  by all three delivery methods) and to WebhookCheckResponseNode (the shape
  returned by Shopify when listing existing subscriptions)
• register.ts:
  • TEMPLATE_GET_HANDLERS now queries the filter field from Shopify
  • buildHandlerFromNode reads filter from the existing subscription node
  • areHandlerFieldsEqual compares filter when deciding whether an update
    is needed
  • buildMutation serializes filter into the GraphQL mutation params
• __tests__/responses.ts: Added webhookCheckResponseWithFilter fixture
• __tests__/register.test.ts: Three new test cases:
  • Sends filter in a create mutation when the handler has it set
  • Triggers an update mutation when the filter value changes
  • Skips the mutation entirely when the filter is unchanged
• docs/reference/webhooks/addHandlers.md: Documented the new filter
  parameter with a link to the Shopify API reference

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used pnpm changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)