Support AgentFacts DataFacts registration

[arnabdastidar]

Summary

• add dependency-free AgentFacts helpers for registry payload construction
• include optional AgentFacts metadata in adapter registration via constructor, setter, and AGENT_FACTS_PATH
• validate the NANDini data_facts_url bridge field for http(s) and df:// pointers
• harden file-loaded AgentFacts against malformed null and non-standard JSON values
• document and test the registration payload behavior

Why

Agents can now advertise the Data Facts document associated with their service through AgentFacts registration. This addresses the metadata gap raised in #21 and creates the adapter-side bridge needed for NANDini-style data discovery.

Validation

• python -m pytest tests/test_agentfacts.py -q
• python -m py_compile nanda_adapter/core/agentfacts.py tests/test_agentfacts.py nanda_adapter/core/agent_bridge.py nanda_adapter/core/nanda.py
• git diff --check
• pip check in an isolated runtime environment after installing the adapter dependencies
• booted a local NANDA bridge on http://localhost:7600/a2a and verified an A2A /help round-trip with python-a2a

Review Hardening

• local bridge startup no longer reads AGENT_FACTS_PATH when registration is disabled
• failed AgentFacts file loading during registration no longer prevents the bridge from starting
• JSON null, NaN, and Infinity values are rejected during AgentFacts validation