fix(confidential): restore bounded MPT decryption

[kpitapeersyst]

fix(confidential): restore bounded MPT decryption

Description

This PR restores the bounded decryption contract required by the current vendored mpt-crypto library. It validates inclusive amount ranges and propagates them through the low-level CGo bindings, ElGamal helpers, and high-level confidential transaction builders.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

Changes

• Add inclusive amount-range validation to ElGamal decryption.
• Update the CGo and !cgo DecryptAmount contracts to accept lower and upper bounds.
• Propagate decryption ranges through the send and convert-back builders.
• Add coverage for valid, invalid, and out-of-range decryption attempts.
• Document bounded decryption behavior and its linear search cost.

Tests

• CGO_ENABLED=1 CC=gcc go test ./confidential/...
• CGO_ENABLED=0 go test ./confidential/...

Notes

• This PR targets the temporary confidential-transfers integration branch.
• The exported decryption APIs now require explicit amount bounds.
• Changelog updates are intentionally deferred until the integration branch stabilizes.