Throw CborContentException for dangling tag at end of root-level sequence

[MuhammadBilal64]

Fixes #135191

CborReader now throws CborContentException when a root-level value sequence (AllowMultipleRootLevelValues) ends with a tag that is not followed by a data item, instead of reporting Finished. This applies to both single-buffer and incremental (SlideData) readers, so equivalent states now throw the same exception type.

Changes:

• PeekStateCore: throw at the end of a root-level sequence if a tag is pending
• PeekInitialByte: same check before the end-of-sequence guard, which covers the case where the tag is the only token consumed (SlideData rebases the offset to zero)
• Updated PeekState_DanglingTagAtEndOfRootSequence, which asserted the old Finished behavior, and added a test for the tag-only case

Note: this changes shipped behavior. PeekState() previously returned Finished for a dangling root-level tag.

Testing: I verified the scenarios from the issue (before and after the change) with a scratch console project that compiles the library sources. I couldn't run the full System.Formats.Cbor test suite locally, so I'm relying on CI for that.

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