[Swift] Verify enums using their serialized raw type

[HappyCode0806]

Swift can store a no-payload enum in fewer bytes than its declared raw scalar type. FlatBuffers serializes the raw scalar width, so using the Swift enum type for verifier bounds checks can validate fewer bytes than generated accessors read or write.

This change verifies Self.T, the generated enum's serialized raw type, and adds a regression test for a UInt64-backed enum. The test checks that a truncated value is rejected at the 8-byte boundary while a complete value continues to verify.

Security report: Google Issue Tracker 560859506 (restricted).

Testing:

• Vulnerable-line negative control fails with the expected missing-error assertion.
• Fixed targeted regression passes 4/4.
• Full Swift test suite passes: 95 tests in 15 suites.

Command:

DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcrun swift test

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