Fix/v9 test issues

[swmal]

Make the test suite safe for parallel execution

EPPlusTest runs with MSTest parallelization at method level. This PR fixes the intermittent failures this caused.

Library fix

• StructureKeys: a static lookup dictionary was filled lazily without locking. Concurrent first use from separate packages could corrupt it, or silently return null data types for cell picture keys. The table is now built once in the type initializer and only read afterwards.

Tests

• Added [DoNotParallelize] to classes that share a static workbook or change process-wide state (license, global configuration).
• Made output file names unique where tests wrote to the same file.
• Removed a test that set a commercial license key globally.
• SaveAndCleanup now includes the inner exception in the message (MSTest hides it for ClassCleanup failures) and disposes the package when saving fails.
• Added StructureKeysTests: table content, a guard against mutable static fields, and a parallel cell picture smoke test.

Verification

Full suite (~6,700 tests) green twice per target framework (net8.0, net481).