fix: Add defensive error check to mpt_get_send_context_hash

[ManimaranRepos]

Summary

Adds defensive error checking to the getSendContextHash() function to properly handle failures from the underlying mpt_get_send_context_hash() C library function.

Changes

• Add error check to getSendContextHash() in ConfidentialTransfer.cpp
• Throw runtime error if context hash computation fails
• Add period to tecBAD_PROOF error message for consistency

Fixes #7651

[github-actions]

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