fix: honor retry-after in provider fetchers

[RerankerGuo]

Description

Fixes #1620.

LLM and embedding provider fetchers now honor the upstream Retry-After header before retrying transient HTTP responses. The parser supports integer seconds and HTTP-date values, and falls back to the existing exponential backoff when the header is absent or invalid.

Related Issue (Required): Fixes #1620

Type of change

• 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 not work as expected)
• Documentation update

How Has This Been Tested?

• cd apps/memos-local-plugin && npm test -- --run tests/unit/llm/fetcher.test.ts tests/unit/embedding/fetcher.test.ts passed with 22 tests.
• cd apps/memos-local-plugin && npm run lint passed.
• make format could not run because poetry is not installed: make: poetry: Command not found.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation, if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules