Return ActionResult<T> instead of bare string from Navigation/generator endpoints

[wrigjl]

Context

Several controller actions return a bare string/String (or bool) instead of IActionResult/ActionResult<T>. These actions hand-serialize their payload with JsonSerializer.Serialize(...) and then return the resulting string.

Returning a bare string has two concrete consequences (both verified against a running instance):

1. Wrong content type. A string return goes through ASP.NET Core's StringOutputFormatter, so the response is served as Content-Type: text/plain; charset=utf-8 — even though the body is JSON. Compare with ProblemProvider.*, which uses Content(..., "application/json") and correctly returns application/json.
2. No error results. With a string return type the action cannot return BadRequest(...)/NotFound(...). Endpoints that take a problem name currently return an empty [] on unknown/invalid input instead of a proper 4xx.

Proposed change

Convert the affected actions to ActionResult<T> and return theObject; rather than JsonSerializer.Serialize(...), letting the framework serialize to application/json. Add BadRequest(...)/NotFound(...) on invalid input where a problem/reduction name is accepted.

Guideline going forward:

Use	When
ActionResult<T>	One well-defined success payload T, possibly plus error results. Swagger infers the 200 body; framework serializes as application/json.
IActionResult / non-generic ActionResult	No single success type, or returning a raw ContentResult/FileResult.
bare string/bool/POCO	Avoid.

Endpoints that need attention

Controller	Route	Method
ALL_ProblemsRefactorController	GET Navigation/ALL_ProblemsRefactorController	getDefault()
NPC_ProblemsRefactorController	GET Navigation/NPC_ProblemsRefactorController	getDefault()
P_ProblemsRefactorController	GET Navigation/P_ProblemsRefactorController	getDefault()
NPHard_ProblemsRefactorController	GET Navigation/NPHard_ProblemsRefactorController	getDefault()
NPC_NavGraph	GET Navigation/NPC_NavGraph/availableReductions	getConnectedProblems(chosenProblem) — takes a problem name; can't 400/404
NPC_NavGraph	GET Navigation/NPC_NavGraph/reductionPath	getPaths(reducingFrom, reducingTo) — takes problem names
Nav_Visualizations controller	GET Navigation/{controller}	getDefault(chosenProblem, problemType?)
Nav_Reductions controller	GET Navigation/{controller}	getDefault(source?, target?)
Nav_Verifiers controller	GET Navigation/{controller}	getDefault(chosenProblem, problemType?)
Nav_Solvers controller	GET Navigation/{controller}	getDefault(chosenProblem, problemType?)
ProblemGeneratorController	GET ProblemGenerator/UndirectedGraph	getUndirectedGraph(n, density, k)
ProblemGeneratorController	GET ProblemGenerator/DirectedGraph	getDirectedGraph(n, density, k)
ProblemGeneratorController	GET ProblemGenerator/Sat3	getSat3(n, c)

Also worth fixing while here: the ProblemGeneratorController actions are annotated [ProducesResponseType(typeof(string[]), 200)] but return a single instance string, not an array.

Already correct (no change needed)

• ProblemProvider.* — IActionResult; mixes Content(..., "application/json") with BadRequest(...), no single T.
• ContributerProfile.* — IActionResult with Ok/NotFound.
• ProblemTemplate.* — ActionResult returning file downloads (no T).