Add optional base_url input to support GitHub Enterprise Octokit endpoints

[Copilot]

Octokit currently defaults to https://api.github.com, which breaks API calls against GitHub Enterprise Server unless baseUrl is set explicitly.
This PR introduces an action-level configuration for API base URL and threads it through the sub-actions that instantiate Octokit.

• New action input

  • Added optional base_url to root action.yml.
  • Intended format for GHE: https://HOSTNAME/api/v3.

• Input propagation through composite action

  • Forwarded inputs.base_url from the root action into:
    • .github/actions/file
    • .github/actions/fix

• Octokit configuration updates

  • In file/src/index.ts and fix/src/index.ts, read base_url via core.getInput('base_url', {required: false}).
  • Pass baseUrl into new OctokitWithThrottling({...}).
  • Keep existing behavior unchanged when base_url is omitted.

• Docs update

  • Added base_url to README action input table and workflow example.

• Focused coverage

  • Added unit tests for both file and fix action entrypoints to assert:
    • explicit base_url is passed as Octokit baseUrl
    • omitted base_url leaves Octokit on default API base URL behavior

# workflow usage
- uses: github/accessibility-scanner@v2
  with:
    token: ${{ secrets.GH_TOKEN }}
    base_url: https://ghe.example.com/api/v3

const baseUrl = core.getInput('base_url', {required: false}) || undefined

const octokit = new OctokitWithThrottling({
  auth: token,
  baseUrl,
  throttle: { ... },
})

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[JoyceZhu]

Suggested change

	description: "Optional base URL for the GitHub API (for example, 'https://HOSTNAME/api/v3' for GitHub Enterprise Server)"
	description: "Optional base URL to pass into Octokit for the GitHub API (for example, `https://YOUR_HOSTNAME/api/v3` for GitHub Enterprise Server)"

[Copilot]

Pull request overview

Adds a new optional base_url input to the top-level composite action and wires it through to the file and fix sub-actions so Octokit can target GitHub Enterprise Server (or other non-default GitHub API endpoints).

Changes:

• Added optional base_url input to the root action.yml and forwarded it into the file and fix steps.
• Added base_url inputs to .github/actions/file and .github/actions/fix action metadata and passed the value into Octokit construction in both entrypoints.
• Documented the new input in the root README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
action.yml	Defines base_url input and forwards it to file/fix sub-actions in the composite workflow.
README.md	Documents base_url in the usage example and inputs table.
.github/actions/file/action.yml	Declares base_url as an optional input for the file action.
.github/actions/file/src/index.ts	Reads base_url and passes it as Octokit baseUrl.
.github/actions/fix/action.yml	Declares base_url as an optional input for the fix action.
.github/actions/fix/src/index.ts	Reads base_url and passes it as Octokit baseUrl.

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[helen]

Do we need the key here, even just to be consistent? (I am also overthinking the quotes oops)

Suggested change

	baseUrl,
	"baseUrl": baseUrl,