.NET pipeline gaps in CI trigger settings

## Summary

CI triggers are not enabled for the .NET management-plane SDK pipelines. Nearly all `ci.mgmt.yml` files in `azure-sdk-for-net` are set to `trigger: none`, so merging a PR to `main` does not automatically queue the corresponding pipeline. This blocks automatic release pipeline runs on merge and requires manual pipeline queuing today.

## Impact

- Release pipelines are not triggered automatically when a PR merges to `main`.
- The same gap applies to the `hotfix/*` and `release/*` branches.
- Releases depend on manual intervention, which is slower and easy to miss.

## Proposal

Add a `trigger` block to each `ci.mgmt.yml`, scoped by path so that only relevant changes queue each pipeline:

```yaml
trigger:
branches:
include:
- main
- hotfix/*
- release/*
paths:
include:
- sdk/apicenter/Azure.ResourceManager.ApiCenter
```

## Implementation notes

A few details worth calling out for anyone picking this up, based on doing the pass across the repo:

- **246 `ci.mgmt.yml` files** are affected; all but `sdk/resourcemanager/ci.mgmt.yml` were `trigger: none`.
- **Path filters must cover every artifact the pipeline builds, not just `Azure.ResourceManager.*`.** Around 60 management pipelines also build an `Azure.Provisioning.*` package, and those packages are not built by the data-plane `ci.yml`. If their paths are omitted, changes to them would build and release nothing.
- **Several services build multiple packages** from one pipeline (for example `alertsmanagement` with 6, `resources` with 5, `monitor` and `kubernetesconfiguration` with 4 each). Each needs its own path entry.
- **Do not derive the path purely from the artifact `name`.** At least one artifact name does not match its directory casing: `sdk/redhatopenshift` declares `Azure.ResourceManager.RedHatOpenshift` while the directory on disk is `Azure.ResourceManager.RedHatOpenShift`. Azure Pipelines path filters are case-sensitive, so validate against the real directory rather than the declared name. Note that `Test-Path` on Windows is case-insensitive and will not catch this.

## Related

- Repo-wide pass for `azure-sdk-for-net`: Azure/azure-sdk-for-net#62806

## Follow-up

The scaffolding template at `eng/templates/Azure.ResourceManager.Template/content/ci.mgmt.yml` also needs the trigger block so newly onboarded management libraries get it by default. That change requires corresponding scaffolding script updates (the template uses `LowercaseProviderShortName` and `Azure.ResourceManager.Template` placeholders that must be substituted into the path filter), so it is intentionally excluded from the PR above and should be handled separately.