Add DNS-AID SVCB resolver for cloud endpoint discovery

[IngmarVG-IB]

Summary

Adds a dns_aid resolver module to extension/llm/ enabling ExecuTorch edge agents to resolve cloud inference endpoints (vLLM, Ray Serve, remote MCP servers) via DNS-AID SVCB lookups (RFC 9460) rather than hard-coded URLs.

• DnsAidResolver.resolve() queries the _agents subdomain and returns (host, port) of the best-matching endpoint
• Optional min_context_len capability filter for endpoints publishing context_len:<int> capabilities
• Falls back to a static fallback_url when no SVCB records are found
• Zero overhead when dns-aid is not installed (lazy import guard — DnsAidResolverConfig works standalone)
• Client-only: ExecuTorch does not publish DNS records

New files:

• extension/llm/dns_aid/ — resolver module (config.py, resolver.py, __init__.py)
• extension/llm/dns_aid/test_dns_aid_resolver.py — 9 unit tests
• docs/llm/dns_aid.md — usage guide

Modified:

• pyproject.toml — adds [project.optional-dependencies] entry: pip install executorch[dns-aid]

Test plan

• python -m pytest extension/llm/dns_aid/test_dns_aid_resolver.py -v — 9/9 pass
• Verified DnsAidResolverConfig importable without dns-aid installed
• Verified DnsAidResolver() raises helpful ImportError without dns-aid
• Verified resolve_sync() raises clear error when called inside a running event loop
• Formatted with black; flake8 clean

cc @larryliu0820 @mergennachin @cccclai @helunwencser @jackzhxng

[pytorch-bot]

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[IngmarVG-IB]

@pytorchbot label "release notes: none"

[IngmarVG-IB]

@pytorchbot label "release notes: misc"