Fix startup failure: pass continue-on-error to the reusable workflow as an input

[jaraco]

Problem

Since #199 refactored main.yml to call the reusable test-suite.yml, the test job carries a job-level continue-on-error:

  test:
    uses: ./.github/workflows/test-suite.yml
    with: ...
    continue-on-error: ${{ matrix.python == '3.15' }}

continue-on-error is not a permitted key on a job that calls a reusable workflow. GitHub rejects the entire file at validation time:

Invalid workflow file: .github/workflows/main.yml
(Line: 47, Col: 5): Unexpected value 'continue-on-error'

The run fails at startup (0s, no jobs, no checks) before anything executes. This has taken CI dark on skeleton itself (red on every commit since #199) and on every downstream project that has merged the skeleton since 2026-07-12 — the PR shows no checks running at all.

Fix

Move the setting into the reusable workflow as a boolean continue-on-error input, applied to its run job, and pass it through from the caller via with:. This:

• restores a valid workflow file (jobs start again),
• preserves the "allow 3.15 (pre-release) to fail" behavior,
• keeps the version-specific 3.15 logic in main.yml where the matrix lives, so test-suite.yml stays version-agnostic.

The collateral caller doesn't pass the input, so it defaults to false — no behavior change there.

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