Groom code (remove dead code, increase coverage)

[sarroutbi]

This series of commits significantly refactors the evidence collection logic in the push model agent and substantially increases the test coverage for the struct_filler and state_machine modules.

The primary motivation is to improve code quality by removing dead code paths and ensuring the core state transition logic is robust and well-tested.

• Key Changes:

  • struct_filler.rs Refactoring (Commit 1/3):

    • Removed Dead Code: The unused FillerFromFile and FillerFromCode structs, along with their associated helper functions, have been completely removed. This significantly simplifies the module's public API and internal logic.

    • Simplified Logic: The get_filler_request function has been streamlined. It now has a single decision point: it returns a FillerFromHardware if a TPM ContextInfo is provided, otherwise it falls back to the TestingFiller. This makes the code's intent clearer and easier to maintain.

  • Increased Coverage for struct_filler.rs (Commit 2/3):

    Following the refactoring, a comprehensive suite of unit tests has been added to cover the remaining logic in struct_filler.rs. The new tests verify:

    • Correct dispatcher behavior in get_filler_request for both Some(Context) and None cases.

    • The default behavior of the TestingFiller.

    • Error handling paths in FillerFromHardware, such as when parsing a malformed verifier response or when underlying hardware/TPM access fails.

  • Increased Coverage for state_machine.rs (Commit 3/3):

    • A new suite of integration-style tests has been added for the StateMachine.

    • These tests use a mock registration module (injected via conditional compilation) and a mock AttestationClient to test the state machine's flow control in isolation from network and hardware dependencies.

    • The tests now cover the "happy path" of the run() method from Unregistered to Complete, as well as failure transitions if the negotiation or attestation phases fail.

[codecov]

Codecov Report

Attention: Patch coverage is 34.14634% with 27 lines in your changes missing coverage. Please review.

Project coverage is 58.92%. Comparing base (b2e7fba) to head (dc05d3d).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/struct_filler.rs	19.35%	25 Missing ⚠️
keylime-push-model-agent/src/attestation.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.92% <34.14%> (+1.04%)	⬆️
upstream-unit-tests	58.92% <34.14%> (+1.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
keylime-push-model-agent/src/state_machine.rs	45.66% <100.00%> (+35.49%)	⬆️
keylime-push-model-agent/src/attestation.rs	61.06% <0.00%> (+16.03%)	⬆️
keylime-push-model-agent/src/struct_filler.rs	23.61% <19.35%> (+0.24%)	⬆️

... and 7 files with indirect coverage changes

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