fix: resolve MCP endpoint path for DNS-discovered agents (v0.12.1)

[iracic82]

Summary

• MCP endpoint path resolution — DNS SVCB records give host:port but not the sub-path where MCP JSON-RPC handlers live (e.g., /mcp). New resolve_mcp_endpoint() discovers the correct path via /.well-known/agent.json, with /mcp convention fallback. Applied automatically in call_mcp_tool() and list_mcp_tools().
• Default A2A timeout bumped from 30s to 60s — Agents performing multi-step analysis (DNS, DNSSEC, TLS probing) need more than 30s. The old default caused Claude Desktop to timeout on first attempt.
• Improved MCP tool descriptions — list_published_agents now clarifies it requires backend API credentials; discover_agents_via_dns clarifies it works on any public domain with no credentials.

Changes

src/dns_aid/core/invoke.py

• New resolve_mcp_endpoint() — 3-tier path resolution: explicit path → agent.json endpoints → /mcp convention
• call_mcp_tool() and list_mcp_tools() call resolve_mcp_endpoint() before invoking

src/dns_aid/mcp/server.py

• send_a2a_message default timeout: 30s → 60s
• list_published_agents docstring: clarifies credential requirement
• discover_agents_via_dns docstring: clarifies no credentials needed

Version bump

• pyproject.toml, __init__.py, CITATION.cff: 0.12.0 → 0.12.1
• CHANGELOG.md: v0.12.1 entry

Test plan

• 765 unit tests passing
• Lint + format clean (ruff check + ruff format --check)
• Verified live: resolve_mcp_endpoint("https://booking.highvelocitynetworking.com:443") → discovers /mcp from agent.json
• Verified live: list_mcp_tools and call_mcp_tool with host:port-only endpoint now succeed
• Verified live: send_a2a_message to security-analyzer succeeds with 60s default