feat: add async job mode to Docling Serve converter

[2830500285]

Summary

• Add an async job mode for DoclingServeConverter that submits conversions to Docling Serve's async endpoints and polls until completion.
• Support async jobs for both URL sources and uploaded file/ByteStream sources.
• Preserve the existing synchronous mode as the default and cover serialization, failure handling, and sync/async execution paths with tests.

Closes #3345

Validation

• SETUPTOOLS_SCM_PRETEND_VERSION=0.0.0 uvx hatch run fmt-check src/haystack_integrations/components/converters/docling_serve/converter.py src/haystack_integrations/components/converters/docling_serve/init.py tests/test_converter.py
• SETUPTOOLS_SCM_PRETEND_VERSION=0.0.0 uvx hatch run test:unit tests/test_converter.py
• SETUPTOOLS_SCM_PRETEND_VERSION=0.0.0 uvx hatch run test:types

[github-actions]

Coverage report (docling_serve)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/docling_serve/src/haystack_integrations/components/converters/docling_serve
converter.py					212, 219-226, 231-232, 321-322, 332-334, 357-358, 371-372, 375-390, 428-429, 439-441, 454-456, 565, 588-589
Project Total

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