fix: handle upload already_exists races across workflows

[api2062]

Description

Fixes a race condition when multiple workflows upload an asset with the same filename to the same release tag.

When overwrite_files is enabled (default), concurrent runs can still hit:

Validation Failed: {"resource":"ReleaseAsset","code":"already_exists","field":"name"}

because another workflow may upload between the initial asset check and upload call.

What changed

• In upload():
  • If upload fails with 422 + already_exists, the action now:
    1. refreshes the release asset list,
    2. finds the conflicting asset by aligned name,
    3. deletes it,
    4. retries the upload once.
• Added a regression test covering this path:
  • retries upload after deleting conflicting asset on 422 already_exists race.

Fixes #740

Validation

• npx vitest run __tests__/github.test.ts
• npm run typecheck
• npm run build

[chenrui333]

Thanks @api2062!

[chenrui333]

#767