Move the test runner to Microsoft.Testing.Platform

[kkdev92]

xunit.v3 4.0 drops the VSTest path on the .NET 10 SDK, which is why Dependabot's bump (#35) could
not be merged. Taking it means opting into the MTP mode of dotnet test.

What changes

• global.json gains test.runner
• dotnet test takes --project / --solution rather than a bare path, and filters become MTP
  options after a --: --filter-not-trait Category=Package in place of
  --filter "Category!=Integration&Category!=Package"
• xunit.runner.visualstudio is the VSTest adapter and is no longer referenced
• the same rewrite in ci.yml, release.yml, integration.yml, and in every command printed in
  the README, CONTRIBUTING, the pull request template and docs/compatibility.md

The one behaviour that changes

The integration project is no longer filtered out of the ordinary test pass. Under MTP an assembly
that matches no tests fails the run rather than printing a note, so excluding it would turn CI
red. It is discovered and skips itself for want of a credential — which also means a break in that
project surfaces in CI rather than at release.

Verification

Compared against the pre-migration run per assembly, not on the total, since a total can match
while assemblies shift:

assembly	before	after
Authentication.Tests	107	107
CodeGen.Tests	105	105
ContractTests	62	62
Tests	177	177
Webhooks.Tests	64	64
IntegrationTests	excluded	1 skipped
package pass	39	39

dotnet format and the generated-source verification are unchanged and green.

The three failure modes the design rests on were reproduced before relying on them:

	result
also exclude Category=Integration	exit 8, one assembly errors — this is why it is not excluded
drop the filter entirely	555 instead of 516, so the filter is doing work
mistype the option name	exit 5 with zero tests, not a quiet pass

That last one matters most: a filter that is silently ignored looks exactly like a filter that
works. --filter "Category!=Package" is accepted and ignored under MTP, which is the trap this
was checked against.

This also removes the Dependabot ignore rule added in #39, which named this migration as its exit
condition.

🤖 Generated with Claude Code