.NET: Python: implement feature-usage User-Agent telemetry

[eavanvalkenburg]

Implement the Python half of #7240 following ADR-0033 and SPEC-004.

The implementation should:

• maintain a process-global, monotonic 128-bit feature mask;
• mark all Python v1 registry capabilities at first meaningful activation;
• emit the live mask only through approved first-party request paths and origins;
• preserve SDK transport defaults, redirect safety, and caller-owned client boundaries;
• provide the dedicated mask-only opt-out while retaining Python's existing whole-User-Agent opt-out;
• validate exact registry parity, complete coverage, range, and zero overlap;
• report the installed agent-framework-core version in the Agent Framework User-Agent.

The .NET implementation remains tracked by #7240.

[eavanvalkenburg]

Superseded by #7421, which is now the canonical Python tracking issue linked from PR #7420.