Sync armstrong-numbers tests with problem specifications

[ManasDasri]

Syncs the armstrong-numbers exercise with the latest canonical test data from problem-specifications.

Changes

• Verified the existing test suite against the canonical data.
• Added intentional exclusions for the two canonical big-integer cases that exceed the range of the exercise's Integer parameter.
• Documented the reason for both exclusions in .meta/tests.toml.

Verification

• pwsh ./bin/test.ps1 armstrong-numbers
• All 9 supported tests pass successfully.

Closes #421

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