chore: pin third-party GitHub Actions to commit SHAs

[mahangu]

Pins third-party GitHub Actions in this repo to immutable commit SHAs.

This is a draft PR for review before merging. It was prepared with agent assistance and manually verified.

Tracking: DEVPROD-1072

Repo-level summary:

• Pinned distinct third-party action refs in this PR: 2
• Repo-level unpinned usage count from the trunk recheck: 7
• Dependabot GitHub Actions coverage: created (.github/dependabot.yml)

Verification commands:

# oven-sh/setup-bun # v2.2.0 -> 0c5077e51419868618aeaa5fe8019c62421857d6
gh api repos/oven-sh/setup-bun/commits/v2.2.0 --jq '.sha'
# expected: 0c5077e51419868618aeaa5fe8019c62421857d6

# shivammathur/setup-php # 2.37.1 -> 7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc
gh api repos/shivammathur/setup-php/commits/2.37.1 --jq '.sha'
# expected: 7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc