Self-describing instance/certificate formats; structured 400 on parse error

[wrigjl]

Adds two new fields to IProblem and turns parse failures on /verify and /problemInstance into structured 400 responses that quote the expected format back to the caller.

New fields on IProblem (default-implemented as "" so existing problems compile unchanged):

• string instanceFormat — a short descriptive sentence with an embedded concrete example of the instance string the problem expects
• string certificateFormat — same shape, but describes what the verifier accepts as a certificate

Populated on SAT3 and CLIQUE as a pilot. Other problems return empty strings until backfilled.

Two new exception types in Interfaces/ParseExceptions.cs:

• ProblemParseException — thrown by a problem's instance constructor on malformed input. Carries the offending string.
• CertificateParseException — thrown by a verifier on malformed certificate input. Carries the IProblem instance so the controller can read its certificateFormat.

SAT3 and CLIQUE constructors throw ProblemParseException on shape errors instead of silently constructing a half-populated problem object. SAT3Verifier and CliqueVerifier throw CertificateParseException instead of silently dropping malformed assignments (previously, lowercase 'true'/'false' in a SAT3 certificate would parse to an empty true-literal list and the verifier would just return false — no diagnostic).

IVerifier.verify default impl unwraps TargetInvocationException from Activator.CreateInstance so a ProblemParseException thrown from T(string) surfaces with its real type to the controller layer.

POST /ProblemProvider/verify and POST /ProblemProvider/problemInstance now return IActionResult. On parse failure they return HTTP 400 with body:

{ "error": "instance_parse_error" | "certificate_parse_error",
"problem": "",
"expected": "",
"received": "",
"detail": "" }

The "expected" field is the same string exposed by /ProblemProvider/info on instanceFormat/certificateFormat — single source of truth for the format hint. Success responses are unchanged (same JSON-string body, HTTP 200).

Verified live: the canonical SAT3 gut-check certificate "x1=true,x2=true,x3=false,x4=true" now returns 400 with the SAT3 certificateFormat in expected and "assignment 'x1=true' has value 'true'; expected True/False (capitalized) or T/F" in detail.

Backfilling the remaining ~41 problems and extending the validate-and-throw pattern to their verifiers is a natural follow-up.