tests: MC/DC decision coverage for dh.c and dsa.c

Adds tests/api DecisionCoverage functions closing MC/DC gaps identified
by the per-module campaign (iso26262/mcdc-per-module):

dh.c:    BEFORE 51/173 (29.48%) -> AFTER 107/173 (61.85%), union across
         6 native variants (sp_default, sp_dh, sp_dh_nonblock,
         small_stack, no_dh186, validate_keygen). New coverage: wc_DhSetKey
         family bad-args and the FFDHE-table primality fast-path, named-key
         helpers (SetNamedKey/GetNamedKeyParamSize/CopyNamedKey/CmpNamedKey),
         wc_DhGenerateKeyPair/GeneratePublic bad-args plus a multi-group
         (2048/3072/4096) generate+agree(+ct) round trip, WC_DH_NONBLOCK's
         incremental state machine, wc_DhImportKeyPair/ExportKeyPair,
         wc_DhCheckPubKey(_ex)/wc_DhCheckPrivKey(_ex)/wc_DhCheckKeyPair
         (+ WOLFSSL_VALIDATE_DH_KEYGEN), wc_DhGenerateParams/
         wc_DhExportParamsRaw, and CheckDhLN's divLen==224/256 MC/DC pair.

dsa.c:   BEFORE 34/110 (30.91%) -> AFTER 51/110 (46.36%), union across
         4 native variants (default, invmod_ct, fastmath, small_stack).
         New coverage: wc_DsaSign_ex/wc_DsaVerify_ex digestSz bad-argument
         checks and the q==1 qMinus1-iszero guard, individual single-operand
         NULL-argument combinations for ImportParamsRaw/ExportParamsRaw/
         ExportKeyRaw, and CheckDsaLN's case-2048 divLen==224/256 MC/DC pair.

Both modules build and pass their tests/api group + KATs across every
native variant with zero failures (10/10 variant runs).

Author: danielinux