feat: Add self-hosted mode support to Confluence Handler and API client

[creatorpiyush]

Description

This PR adds first-class support for Confluence Server/Data Center (on-prem) while keeping Cloud behavior unchanged by default.

Resolved runtime KeyError on spaces projection in Server mode: Missing columns like description_view_representation, description_view_value, authorId, createdAt, homepageId, currentActiveAlias are now handled via normalization/placeholder fields before DataFrame projection.

Net Outcome

• Prevents breaking change from the cloud-first flag.
• Preserves existing user configurations via fallback mapping.
• Ensures correct endpoint selection for both Cloud and self-hosted Confluence.
• Keeps Cloud-only capabilities safely restricted in self-hosted mode.

Type of change

(Please delete options that are not relevant)

• ⚡ New feature (non-breaking change which adds functionality)
• 📄 This change requires a documentation update

Verification Process

To ensure the changes are working as expected:

• Test Location: Any self-hosted Confluence
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Additional Media:

• I have attached a brief loom video or screenshots showcasing the new functionality or change.

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[creatorpiyush]

I have read the CLA Document and I hereby sign the CLA

[creatorpiyush]

recheck

[creatorpiyush-nagarro]

I have read the CLA Document and I hereby sign the CLA

[creatorpiyush-nagarro]

recheck

[torrmal]

thank you!! can you please create a pull request in github.com/mindsdb/engine