Validate release signing credentials before building

[YuyuZha0]

The 1.2.14 release failed during GPG signing with Unusable secret key after all 722 tests passed. The configured key's local public certificate expired on 2026-08-11 UTC; renewing the key and replacing the repository's GPG_PRIVATE_KEY secret is required to restore publishing. This PR does not renew the key or change repository secrets.

Add a signing probe before the build, using the tagged POM's gpg.keyname and the same passphrase as Maven, so unusable keys and incorrect passphrases fail early with actionable diagnostics. Replace the short key ID with the full fingerprint of the same key.

Validation:

• actionlint and shell syntax checks passed.
• Executed the workflow's signing step with isolated disposable keys: expired key rejected, renewed key signed and verified, wrong passphrase rejected.
• Maven resolved the configured full fingerprint successfully.
• No production Java code, coverage thresholds, or README changes.

Failure: https://github.com/YuyuZha0/fast-printf/actions/runs/37873070924/job/113635295046

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (938ddac) to head (5f7f75f).
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Additional details and impacted files

@@            Coverage Diff            @@
##               main      #39   +/-   ##
=========================================
  Coverage     96.83%   96.83%           
  Complexity     1128     1128           
=========================================
  Files            59       59           
  Lines          2146     2146           
  Branches        341      341           
=========================================
  Hits           2078     2078           
  Misses           33       33           
  Partials         35       35           

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