Detect truncated or malformed license keys and log a clear message

[jbogard]

Sub-issue of #4659.

Problem

In 3 support cases the key was missing its last few characters, from copy/paste or email formatting. Validation fails with IDX10511: Signature validation failed (or IDX14100/IDX14102 for badly malformed values). That tells the user nothing useful, and each case needed a round trip with support to diagnose.

Stripping whitespace doesn't help here: the key is simply incomplete.

Fix

Before calling ValidateTokenAsync in src/AutoMapper/Licensing/LicenseAccessor.cs (after whitespace is stripped), run a cheap structural check:

• The key has exactly 3 dot-separated segments.
• Each segment contains only base64url characters.
• The signature segment is the expected length for RS256 with the 2048-bit Lucky Penny key: 256 bytes, which is 342 base64url characters.

If the check fails, skip validation and log a clear message at error level, not critical. For example:

The Lucky Penny software license key appears to be incomplete or corrupted (it may have been truncated when copied). Copy the full key again from https://luckypennysoftware.com/account.

Do not include the key itself in the log.

Also, when validation does fail with SecurityTokenInvalidSignatureException, add a hint about copy/paste corruption to the existing message, which today is just "Error validating the Lucky Penny software license key".

Tests

• A key missing 1, 5 or 20 trailing characters → the "incomplete or corrupted" message, and no IDX exception is logged.
• A key with 2 segments or 4 segments → the same message.
• A valid key → unaffected.

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