fix(calling): discover Mobius hosts via U2C catalog

[tejzpr]

🚀 Description, Motivation, and Context

Fixes Mobius service discovery for Webex Calling registration.

The Calling client previously expected Mobius hosts to be available in the WDM device registration response. Current Webex service discovery instead provides Mobius clusters through the authenticated U2C Services v2 catalog. When no hosts were found in WDM, the client fell back to a hardcoded EU Mobius endpoint, which could return HTTP 404 for users in other regions.

This change:

• Fetches the U2C service catalog using GET /catalog?format=U2CV2.
• Uses the U2C URL advertised by WDM when available, with a configurable default URL.
• Extracts Mobius hosts from both array and keyed-object Services v2 payloads.
• Normalizes and deduplicates discovered hosts.
• Preserves the existing fallback behavior when service discovery is unavailable.

📖 Issues and Related PRs

• Issue URL: N/A
• Related PR: N/A

🖼 Screenshots

Not applicable; this change does not modify the UI.

🌮 How This Was Tested

• Ran locally and verified
• Created tests (unit and/or E2E)
• Automated testing unnecessary (chores, doc updates, etc.)

Added unit coverage for:

• The authenticated U2C catalog request path and U2CV2 query parameter.
• Services v2 array and keyed-object payloads.
• URL normalization and malformed URL filtering.
• Case-insensitive Mobius service names.
• Host deduplication across discovery sources.

Ran the full SDK test suite with go test ./....

Required Items Complete

• Unit tests written
• Integration tests written
• Coverage verified
• Functionally tested
• Deployable to production
• Uses feature flags
• README updated
• Build passes

Security

No security feature or issue is addressed by this change. The U2C request uses the existing Webex access token and configured HTTP client; no credentials are logged or persisted.

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