refactor: Consolidate encoding/hashing tests to eliminate DRY violations (#1467)

Replace 7 separate test files (Base64Tests, HexTests, Md5Tests,
Sha1Tests, Sha256Tests, Sha384Tests, Sha512Tests) with a single
EncodingTests.cs file that uses shared module definitions and
organized test regions.

This reduces ~290 lines of duplicated test code while maintaining
all 18 original test cases with identical functionality.

Closes #1455

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst