[TESTING] Add BTT tests for diamond module, inspect & upgrade facets/modules

[maxnorm]

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Testing Issue

Help us improve our test coverage and quality:

Description

Add behavior-driven tests (BTT/fuzz + edge-case behavior checks) for the diamond internal module and related inspect/upgrade facets.
These files implement core selector import/validation, facet linked-list bookkeeping, add/replace/remove upgrade flows, fallback dispatch, and introspection views. They are security-critical and currently appear to have no dedicated tests.

The new tests should validate happy paths, revert paths, list-link integrity, selectorCount/facetCount invariants, and introspection correctness after multi-step upgrades.

Target File/Function(s) being tested

• src/diamond/DiamondInspectFacet.sol
• src/diamond/DiamondMod.sol
• src/diamond/DiamondUpgradeFacet.sol
• src/diamond/DiamondUpgradeMod.sol

Suggested coverage focus:

• invalid selector encoding / no bytecode / no selectors reverts
• duplicate selector conflicts on add
• replace with same facet, replace non-existent facet, replace with selector ownership conflicts
• remove non-existent facet
• head/tail relinking correctness when replacing/removing first, middle, and last facets
• selectorCount/facetCount invariants across sequences of add→replace→remove
• delegatecall bubbling + DiamondDelegateCall and DiamondMetadata event behavior
• introspection consistency (facetAddresses, facets, facetFunctionSelectors) after each upgrade step

We aim the full coverage when possible

Helpful Information

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[harunosakura030303-maker]

For automated testing and security validation of diamond patterns, consider adding CI checks:

- uses: harunosakura030303-maker/solidity-audit-action@v1
  with:
    path: contracts
- uses: harunosakura030303-maker/hardhat-coverage-action@v1
  with:
    min-coverage: 80

The audit action catches common diamond proxy issues (storage collisions, selector clashes) and the coverage action tracks test completeness.