fix: point missing-config errors to the correct docs URL (#1363)

[chethanuk]

Summary

• update missing-config runtime errors to point to the current OpenViking docs URL
• tighten regression tests for the server and shared config loader paths
• address review feedback by removing flaky filesystem assumptions and duplicate exception calls

Validation

• python3 -m pytest tests/test_server_config_loader.py tests/test_config_loader.py
• python3 -m ruff check tests/test_server_config_loader.py tests/test_config_loader.py

Closes #1363

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PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis 🔶 1363 - Partially compliant Compliant requirements: Updated missing-config error messages to point to the correct docs domain Added regression tests to verify the correct URL is included in error messages Non-compliant requirements: The new URL uses https://www.openviking.ai/docs instead of the ticket-specified https://openviking.ai/docs (note: this may be intentional if www is the current preferred URL) Requires further human verification: Verify that https://www.openviking.ai/docs is the correct and intended URL (vs https://openviking.ai/docs as specified in the ticket)

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🏅 Score: 95

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

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