Add unit tests for low-coverage public header helpers

[achamayou]

Problem

The Coverage run 33884132155 at commit d7b19f3 shows several public-header helpers with straightforward missing unit coverage. The coverage-report-html artifact reports line coverage for include/ccf/**.

The opportunities below are limited to pure formatting, parsing, conversion, schema, and small synchronization helpers. They should not require a node, enclave, KV store, or end-to-end test setup.

Opportunities

Priority	Public header	Current line coverage	Obvious unit test	Estimated additional covered lines
1	base_endpoint_registry.h	0/54	Table-test every ApiResult and InvalidArgsReason, including an out-of-range value for each default branch	54
2	rest_verb.h	13/63	Exercise string construction, known/unknown methods, ordering, JSON round-trip/type error, and schema helpers	50
3	tx_id.h	22/63	Table-test valid IDs plus missing separator, zero/invalid/trailing view and seqno, and both JSON error paths	41
4	service/tables/proposals.h	0/37	Format every ProposalState and verify an out-of-range state throws	37
5	crypto/pem.h	32/57	Extend pem_test with vector construction, ordering/empty/hash, JSON array/error cases, and schema helpers	25
6	tx_status.h	29/72	Extend tx_status_test to check tx_status_to_str for all values and its default branch	25
7	crypto/san.h	0/23	Parse IP and DNS SANs, parse a list, and reject an unknown prefix	23
8	crypto/curve.h	0/21	Check each valid curve-to-digest mapping plus invalid and out-of-range curve errors	21
9	ds/locking.h and wait	39/46	Check native_handle() and wake the non-predicate ConditionVariable::wait() from a helper thread	7
10	claims_digest.h	29/35	Exercise schema_name and fill_json_schema	6
11	http_status.h	3/6	Check client-error boundaries around 400 and 500	3

Conservative total: approximately 292 additional covered lines.

Estimation method

The estimate counts currently uncovered executable lines in the exact run's LLVM HTML report that the described tests should execute. It is not the total uncovered size of each header. For tx_status.h, the header has 43 uncovered lines overall, but only the 25 confirmed-zero formatter lines are counted because evaluate_tx_status already has extensive unit tests.

A practical first batch is priorities 1-4: approximately 182 lines from table-driven tests over dependency-free enum/string helpers.