Reject invalid values for boolean inputs #1157

Description:
Boolean inputs are parsed by comparing their uppercased value with TRUE. Any other non-empty value is silently interpreted as false, so mistakes such as verify-signature: ture disable the requested behavior instead of failing clearly. Replace this behavior with strict parsing that accepts only supported boolean values and reports the input name and invalid value.

Apply the behavior consistently to check-latest, force-download, set-default, verify-signature, overwrite-settings, show-download-progress, and problem-matcher. Add unit tests for valid values, defaults, casing, whitespace, and invalid values.

Task version:
v6

Platform:

• Ubuntu
• macOS
• Windows

Runner type:

• Hosted
• Self-hosted

Repro steps:
Configure an input with an invalid boolean value, for example:

- uses: actions/setup-java@v6
  with:
    distribution: temurin
    java-version: '21'
    verify-signature: ture

Expected behavior:
The action fails before installation with a descriptive validation error identifying verify-signature and its invalid value.

Actual behavior:
The invalid value is silently interpreted as false, and installation continues without signature verification.