Exclude eng/ directory from http-client-csharp publish triggers (#9072)

Changes to engineering infrastructure (pipeline configs, build scripts,
rulesets) in `packages/http-client-csharp/eng/` should not trigger
emitter version publishes.

## Changes

Modified `packages/http-client-csharp/eng/pipeline/publish.yml` to
exclude the `eng/` directory from publish triggers:

```yaml
trigger:
  branches:
    include:
      - main
  paths:
    include:
      - packages/http-client-csharp/
    exclude:
      - packages/http-client-csharp/eng/
```

This prevents unnecessary publishes when modifying:
- Pipeline YAML files
- PowerShell build/test scripts  
- Code analysis rulesets and StyleCop configs
- Pipeline images and other eng resources

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Don't publish new emitter version if only changes are to
eng directory</issue_title>
> <issue_description>We should exclude changes to http-client-csharp/eng
path as triggering a publish.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #9071

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: JoshLove-msft <[email protected]>

Author: Copilot

packages/http-client-csharp/eng/pipeline/publish.yml

@@ -5,6 +5,8 @@ trigger:
   paths:
     include:
       - packages/http-client-csharp/
+    exclude:
+      - packages/http-client-csharp/eng/
 
 pr: none