test: prove real handler output conforms to every declared output schema

Extends TestIssueReadOutputValidatesAgainstDeclaredSchema to the remaining
six tools, taking method coverage from 3 of 28 to 28 of 28. Each tool gets
its own file; each subtest runs the real handler against the existing mock
harness, then validates the emitted payload against the schema the tool
advertises.

Validating the text block is equivalent to validating what a client
receives, because the structured-content mirror publishes those exact bytes
as structuredContent.

Beyond the happy path, the tests cover the cases that actually exercise a
schema's required sets: empty collections, sparse objects with every
optional field absent, and — for actions_get — a different single surviving
key per method, so a non-first branch of each anyOf is exercised rather than
always the first.

Fixes a latent bug the tests surfaced. go-github decodes GetWorkflowRunByID,
GetWorkflowJobByID and GetWorkflowRunUsageByID into a *T, so a 200 carrying
a null body leaves the pointer nil and json.Marshal emits the literal
`null`, which cannot validate against an object-rooted schema. Same class as
the sub-issues bug already fixed here; these three now report the anomaly
instead of returning a payload no caller can use.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: olaservo

pkg/github/actions.go

@@ -802,6 +802,12 @@ func getWorkflowRun(ctx context.Context, client *github.Client, owner, repo stri
 		return ghErrors.NewGitHubAPIErrorResponse(ctx, "failed to get workflow run", resp, err), nil, nil
 	}
 	defer func() { _ = resp.Body.Close() }()
+	// go-github decodes into a *WorkflowRun, so a 200 carrying a null body
+	// leaves it nil and json.Marshal would emit the literal `null`. Report the
+	// anomaly rather than returning a payload no caller can use.
+	if workflowRun == nil {
+		return utils.NewToolResultError("workflow run response was empty"), nil, nil
+	}
 	r, err := json.Marshal(workflowRun)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to marshal workflow run: %w", err)
@@ -815,6 +821,9 @@ func getWorkflowJob(ctx context.Context, client *github.Client, owner, repo stri
 		return ghErrors.NewGitHubAPIErrorResponse(ctx, "failed to get workflow job", resp, err), nil, nil
 	}
 	defer func() { _ = resp.Body.Close() }()
+	if workflowJob == nil {
+		return utils.NewToolResultError("workflow job response was empty"), nil, nil
+	}
 	r, err := json.Marshal(workflowJob)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to marshal workflow job: %w", err)
@@ -1007,6 +1016,9 @@ func getWorkflowRunUsage(ctx context.Context, client *github.Client, owner, repo
 	}
 	defer func() { _ = resp.Body.Close() }()
 
+	if usage == nil {
+		return utils.NewToolResultError("workflow run usage response was empty"), nil, nil
+	}
 	r, err := json.Marshal(usage)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to marshal response: %w", err)