chore: improve swagger markdown optional fields typing

[cqjjjzr]

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Summary

Fixes #36246

Screenshots

Before:

#### ApiKeyItem

| Name | Type | Description | Required |
| ---- | ---- | ----------- | -------- |
| created_at |  |  | No |
| id | string |  | Yes |
| last_used_at |  |  | No |
| token | string |  | Yes |
| type | string |  | Yes |

#### ApiKeyItem

| Name | Type | Description | Required |
| ---- | ---- | ----------- | -------- |
| created_at | integer |  | No |
| id | string |  | Yes |
| last_used_at | integer |  | No |
| token | string |  | Yes |
| type | string |  | Yes |

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	43.70%	+43.70%
Strict coverage	0.00%	43.22%	+43.22%
Typed symbols	0	22,131	+22,131
Untyped symbols	0	28,830	+28,830
Modules	0	2557	+2,557