fix: support bracketed IPv6 runtime URLs

[xianjianlf2]

Fixes #1137

Summary

• support bracketed IPv6 runtime URLs across Node.js, Python, Go, and .NET
• reject URI forms with path, query, or fragment components for runtime host/port parsing
• add URL parsing regression tests for each supported SDK touched by the fix

Validation

• cd go && go test .
• cd python && uv run pytest test_client.py -q
• cd nodejs && npx vitest run test/client.test.ts -t "URL parsing"
• Not run: cd dotnet && dotnet test test/GitHub.Copilot.SDK.Test.csproj --filter RuntimeConnectionUrlParsingTests (dotnet is not installed in this environment: zsh:1: command not found: dotnet)

[SteveSandersonMS]

The underlying gap is worth fixing, but I think this PR should be narrower so it adds IPv6 without changing the existing endpoint grammar or taking on general URL-validation policy.

A minimal version would be:

• Node: after the existing http(s):// removal, recognize the canonical [ipv6]:port form before falling back to the current host:port parser. Avoid moving all inputs through URL.
• Python: add the same narrow bracketed-form branch and retain the existing parser for all other inputs. Also change _connect_via_tcp from an explicitly IPv4 socket.AF_INET socket to family-independent connection resolution (for example, socket.create_connection). At present this PR parses [::1]:4321, but start() still cannot connect to it because it constructs an IPv4-only socket.
• Go: after removing the existing schemes, use net.SplitHostPort for bracket-prefixed inputs and otherwise retain the current behavior. net/url is not needed for this fix.
• .NET: Uri already parses bracketed IPv6; normalize the parsed host to the unbracketed value if necessary and add regression coverage.
• Java: confirm its existing URI path with a regression test and change production code only if the host passed to Socket remains bracketed.
• Rust: no change; its external transport already accepts host and port as separate typed fields.

The intended new cases are simply:

RuntimeConnection.forUri("[::1]:4321");
RuntimeConnection.forUri("http://[2001:db8::1]:4321");

Tests should cover those forms in each URL-based SDK, plus one Python connector-level test proving the connection path is not forced to AF_INET. Parser-only assertions are insufficient there because they currently pass while the actual connection remains broken.

I would leave path/query/fragment rejection, whitespace normalization, arbitrary-scheme handling, userinfo policy, and unrelated port-validation changes out of this PR. Moving wholesale to general-purpose URL parsers changes which previously accepted inputs succeed or fail and introduces cross-language differences that then require us to define and maintain a much larger URL contract. A narrow bracketed-IPv6 branch fixes the demonstrated user problem, preserves compatibility, and avoids pulling those edge cases into scope.

[SteveSandersonMS]

@xianjianlf2 Does that sound reasonable? I'll mark this PR as draft but please remark as ready to review as appropriate.