fix(schema): declare array items for upsert_event tool to satisfy OpenAI strict mode

[chubes4]

Production fire

Every event ingestion flow on events.extrachill.com has been failing since the wp-ai-client cutover (DM 0.103.x) with:

```
Bad Request (400) - Invalid schema for function 'upsert_event':
In context=('properties', 'occurrenceDates'), array schema missing items.
```

OpenAI's strict tool schema validation requires `items` on every array type. The legacy ai-http-client tolerated the omission silently; wp-ai-client does not (correctly — this was always invalid JSON Schema).

Impact

`occurrenceDates` is a field on the `upsert_event` tool — the AI's primary path for writing events into WordPress. Without it working, no events get ingested:

```
status count recent flows affected
failed - ai_processing_failed N Dice.fm, Squarespace, Universal scrapers
completed_no_items N everything else (dedup says nothing new)
new events created in 4h: 0
```

Fix

1. Declare `items` on `occurrenceDates` in EventSchemaProvider

`occurrenceDates` stores ISO date strings (YYYY-MM-DD), so:

```php
'occurrenceDates' => array(
'type' => 'array',
'items' => array( 'type' => 'string' ), // ← added
'required' => false,
'description' => 'For recurring events: array of specific dates (YYYY-MM-DD)...',
'schema_property' => null,
),
```

2. Harden `fieldsToToolParameters()` to always emit `items` for arrays

Defensive: any future array field that forgets to declare `items` will get a sensible default instead of producing an invalid schema.

```php
if ( 'array' === ( $field['type'] ?? '' ) ) {
$param['items'] = $field['items'] ?? array( 'type' => 'string' );
}
```

Verification

After applying, the generated schema for `upsert_event` validates cleanly against OpenAI's strict mode. Event ingestion flows resume on next pipeline tick.

Out of scope

Several output schemas in DM-events ability declarations (DiceFmTest, EventHealthAbilities, BatchTimeFixAbilities) also have `type: array` without `items`. Those are descriptive-only metadata for ability returns and don't go to OpenAI as tool definitions — they're not blocking production. Worth a separate cleanup PR for completeness.

Refs: extrachill.com production incident May 1 2026 ~19:00 UTC after wp-ai-client cutover deploy.

[homeboy-ci]

Homeboy Results — data-machine-events

Audit

✅ audit — passed

• test_coverage — 1 finding(s)
• Total: 1 finding(s)

Deep dive: homeboy audit data-machine-events --changed-since f371a2a

Tooling versions

• Homeboy CLI: homeboy 0.142.0+5a19d01
• Extension: wordpress from https://github.com/Extra-Chill/homeboy-extensions
• Extension revision: 9d5b873
• Action: Extra-Chill/homeboy-action@v2