Fix WasmTestOnChrome: exclude browser/wasi from gen-debug-dump-docs.py helix post-command (#126789)

## Description

`WasmTestOnChrome` and related WASM CI jobs were failing because
`gen-debug-dump-docs.py` was scheduled as a Helix post-command for all
non-Windows CoreCLR targets, but the script is not present in the
correlation payload for `browser`/`wasi` targets.

### Changes

- **`src/libraries/sendtohelixhelp.proj`**: Added `browser` and `wasi`
exclusions to the non-Windows `HelixPostCommands` condition for
`gen-debug-dump-docs.py`, consistent with the existing pattern already
used for `TestEnvFileName` in the same file:

```xml
<!-- Before -->
<HelixPostCommands Condition="'$(TargetOS)' != 'windows'">

<!-- After -->
<HelixPostCommands Condition="'$(TargetOS)' != 'windows' and '$(TargetOS)' != 'browser' and '$(TargetOS)' != 'wasi'">
```

## Changes

- [x] `src/libraries/sendtohelixhelp.proj`

## Testing

Change is a build/CI pipeline configuration fix; validated by inspection
against the existing pattern in the same file.

Fixes #126706

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: danmoseley <6385855+danmoseley@users.noreply.github.com>
Co-authored-by: Dan Moseley <danmose@microsoft.com>

Author: Copilot

src/libraries/sendtohelixhelp.proj

@@ -271,7 +271,7 @@
         $(HelixPostCommands);
         %HELIX_PYTHONPATH% %HELIX_CORRELATION_PAYLOAD%\gen-debug-dump-docs.py -buildid $(BUILD_BUILDID) -workitem %HELIX_WORKITEM_FRIENDLYNAME% -jobid %HELIX_CORRELATION_ID% -outdir %HELIX_WORKITEM_UPLOAD_ROOT% -templatedir %HELIX_CORRELATION_PAYLOAD% -dumpdir %HELIX_DUMP_FOLDER% -productver $(ProductVersion)
       </HelixPostCommands>
-      <HelixPostCommands Condition="'$(TargetOS)' != 'windows'">
+      <HelixPostCommands Condition="'$(TargetOS)' != 'windows' and '$(TargetOS)' != 'browser' and '$(TargetOS)' != 'wasi'">
         $(HelixPostCommands);
         $HELIX_PYTHONPATH $HELIX_CORRELATION_PAYLOAD/gen-debug-dump-docs.py -buildid $(BUILD_BUILDID) -workitem $HELIX_WORKITEM_FRIENDLYNAME -jobid $HELIX_CORRELATION_ID -outdir $HELIX_WORKITEM_UPLOAD_ROOT -templatedir $HELIX_CORRELATION_PAYLOAD -dumpdir $HELIX_DUMP_FOLDER -productver $(ProductVersion)
       </HelixPostCommands>