Fix DIF holder presentation signing for did:key

[jacksonriding]

Fixes #4195.

Preserves explicit issuer_id when the original presentation definition is reused; honors explicit pres_signing_did before holder auto-detection; skips non-local subject DIDs while discovering a local did:key; and fails explicitly when holder signing cannot determine a local DID instead of returning an unsigned VP.

Validation: 74 affected DIF tests passed, 46 optional BBS tests skipped; Ruff passed; git diff --check passed.

[jacksonriding]

Forgot to sign :P

[swcurran]

The two failing tests look real -- could you @jacksonriding take a look at them? The ruff one should be trivial, but not sure about the PR Tests -- certainly seems to be in the area that you are updating.

@jacksonriding -- I'm assuming that once this PR is merged, it should be cherry-picked and applied to the 1.6.lts and 1.3.lts branches as well? Are you able to do that or should I plan to do it?

[jacksonriding]

Thanks @swcurran. I tracked down and fixed both failures. Ruff was a formatting issue in the new test. The PR test
failures were caused by an incorrectly configured requirement mock and an existing suite-mismatch test that still expected an unsigned VP instead of the new explicit error behavior.

DIF suite now passes all tests with BBS. Ruff also passes.

Will look into backporting 1.6 and 1.3 lts after merge

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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