Improve clarity in mobile scanner workflow issues and PRs (#127568)

## Description

The mobile platform failure scanner has been failing with
permission-denied errors and produced a mix of PRs, issues, and repeated
comments on the same issues across runs. This change makes the workflow
simpler and more predictable.

Now every mobile failure either becomes a draft PR (when there's a
per-test fix like `[SkipOnPlatform]`) or a tracking issue (for product
bugs, native crashes, multi-assembly regressions, or infra problems that
need an owner). The workflow no longer files comments and no longer ends
a run with `noop`.

Other changes:

- Cap raised to 5 PRs and 3 issues per run.
- PRs now only touch test files (`src/libraries/**/tests/**` and
matching `.csproj`); anything else fails PR creation.
- PR and issue bodies use a fixed structure: Reasoning, Impact on
platforms, Errors log, First build it occurred (issues add a Recommended
action section).
- Removed broken references in the prompt (`pwsh Get-CIStatus.ps1`, `gh
search prs`) and dropped `pwsh` from the allowlist.

Validated with one workflow_dispatch run on this branch where it
produced three issues #127563, #127564, #127565.

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kotlarmilos