fix(auth): require secret for internal service mode

[PranavaKCode]

Description

• reads MANTIS_INTERNAL_SERVICE_SECRET into the SDK configuration
• sends X-Internal-Secret with the existing internal-service marker and user ID
• fails before any network request when internal-user mode lacks the required secret
• redacts the internal secret from debug transport logs
• documents and tests the authenticated internal-service contract

Motivation and context

MantisAPI PR #2442 replaced the spoofable marker-only bypass with an authenticated X-Internal-Secret contract. SDK callers using MANTIS_INTERNAL_USER_ID must send that same provisioned secret or their requests fall through to browser authentication.

Fixes #11.

This is intentionally fail-closed: the SDK accepts a provisioned secret but never derives or logs it. Cookie authentication is unchanged.

How has this been tested?

• python -m ruff check .
• uv run --with pytest --with pytest-asyncio pytest -q (71 tests passed)
• focused coverage verifies the valid three-header set, missing-secret failure, environment loading, and debug-log redaction
• git diff --check

Screenshots (if appropriate):

Not applicable; this is an SDK authentication contract change.

Types of changes

• ✅ Bug fix (non-breaking change which fixes an issue)
• ✅ Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project. [REQUIRED]
• Ran ruff check. There are no style warnings. [REQUIRED]
• Ran pytest. All new and existing tests passed. [REQUIRED]
• Recorded a video walkthrough showing the platform changes, testing used to verify the fix, and a brief review of the code changes made in this PR. [REQUIRED]
• My change requires a change to the documentation and I have updated it accordingly.
• I have added tests to cover my changes.

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