Fix Zulip connector schema + links and enable temporal metadata

[ATSiem]

Description

This PR addresses critical issues with the Zulip connector's metadata handling and URL validation:

1. Fixes type mismatches in Document model metadata that were causing indexing failures (enable indexing Zulip Server + Cloud)
2. Improves URL handling for Zulip realm configurations (fix reference links)
3. Enhances metadata fields to support temporal document ordering (enhance RAG)
4. Adds proper timezone handling for message timestamps (enhance RAG)

Key improvements:

• Type-safe metadata handling with proper string conversions
• Robust URL validation with scheme detection and domain normalization
• Enhanced error handling for message link generation
• UTC-aware timestamp handling for temporal consistency

How Has This Been Tested?

1. Connector Initialization:

• Tested with various URL formats (http, https, domain-only)
• Verified proper handling of malformed URLs
• Confirmed domain normalization works correctly

2. Document Creation:

• Verified metadata type conversion and validation
• Tested message link generation across different realm configurations
• Confirmed UTC timestamp handling for messages
• Validated temporal ordering of documents

3. Type Safety:

• Ran mypy type checking
• Verified all metadata values are properly converted to strings
• Confirmed error handling for None values

4. Integration Testing:

• Tested full indexing flow with Zulip Server 8.2+ and Zulip Cloud
• Verified document creation through the ingestion API
• Confirmed temporal RAG pipeline functionality (see example)

Backporting

• This PR should be backported
• Override Linear Check

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[yuhongsun96]

lgtm!