#333 add rejection conditions for signer index 0 for TE constructs

[PropzSaladaz]

Description

This PR addresses inconsistent signer-index validation across the threshold-encryption codebase.

A signer index of 0 is invalid: it is reserved for the interpolation target f(0), while participant shares use indices 1..totalSigners. Previously, several TE constructors and TEDecryptSet accepted 0; the invalid input only failed later when combineShares reached Lagrange interpolation.

This PR:

• rejects 0 in TEPrivateKeyShare, TEPublicKeyShare, and TEDecryptionShare;
• enforces the full 1..totalSigners range in TEDecryptSet;
• adds a defensive zero-index check in lagrangeCoeffs;
• updates affected tests and adds regressions.

This does not change valid threshold-encryption behavior or address a new cryptographic security condition. It makes invalid input fail at the API boundary, with a clear validation error, rather than at merge time.

Fixes #333

[Copilot]

Copilot review overview

🟢 Approval recommended

Validation changes and regressions are covered; only minor formatting nits remain.

Review effort: Lite
Findings: None

What changed in this PR

This PR adds early validation for invalid signer indices across threshold-encryption APIs and interpolation.

Changes:

• Rejects zero and out-of-range signer indices.
• Adds defensive Lagrange interpolation validation.
• Updates regression and compatibility tests.

File	Summary
threshold_encryption/​TEPublicKeyShare.cpp	Validates public-key share indices.
threshold_encryption/​TEPrivateKeyShare.cpp	Validates private-key share indices.
threshold_encryption/​TEDecryptSet.cpp	Enforces the valid signer range.
threshold_encryption/​TEDecryptionShare.cpp	Rejects zero-index decryption shares; two line-wrapping nits noted.
test/​unit_tests_backend.cpp	Adds interpolation and inverse regressions.
test/​test_TE_wrappers.cpp	Updates fixtures and adds constructor regressions.
backends/​interface/​functions.cpp	Rejects zero interpolation indices.

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