Run PR Validation on push to main so status checks appear in branch protection picker

[jevansaks]

Why

The PR Validation workflow currently only triggers on pull_request (and workflow_dispatch), so its check-runs are never recorded against commits on main. GitHub's branch-protection "Status checks that have run recently" autocomplete dropdown is populated from check runs observed on the protected branch's recent commits — since main itself has none of these jobs in its history, the dropdown stays empty and you can't pick Scrape headers: x64/x86/arm64 or Build, test, package to require them.

Verified with:

$ gh api repos/microsoft/win32metadata/commits/main/check-runs
{"total_count":0,"check_runs":[]}

What this change does

Adds a push: branches: [main] trigger (with the same paths-ignore as the pull_request trigger) so the same matrix of jobs runs after every merge to main.

on:
  pull_request:
    branches: [main]
    paths-ignore:
      - ''apidocs/**''
      - ''docs/**''
  push:
    branches: [main]
    paths-ignore:
      - ''apidocs/**''
      - ''docs/**''
  workflow_dispatch:

Effect

• After the first post-merge run, Scrape headers: x64, Scrape headers: x86, Scrape headers: arm64, and Build, test, package will appear in the branch-protection status-check picker at https://github.com/microsoft/win32metadata/settings/branch_protection_rules/23707585 and can be marked required.
• main also gets post-merge validation, catching any breakage that slips through PR validation (e.g. semantic merge conflicts between two PRs).
• No change to PR behavior. The existing concurrency group still groups PR runs by PR number and push runs by github.ref (i.e. refs/heads/main); cancel-in-progress: true already applied to PRs, and applies to main pushes the same way (a newer push will cancel an in-flight main run — acceptable, but happy to switch to cancel-in-progress: false for main if preferred).

Alternatives considered

• Type the check names manually in the branch protection UI — works, but brittle if names change and the dropdown remains empty for future maintainers.
• Migrate to a Repository Ruleset that requires a workflow by file path — cleanest long-term, but a larger change. This PR is the minimal fix.