fix(functions): treat invoke 2xx as success regardless of body shape (VOL-766)

[connorckong]

Summary

• Fixes VOL-766: volcano functions invoke reported a false error on genuine 2xx responses whenever the function handler returned a non-object body (plain text, HTML, empty, or non-object JSON).
• Root cause (validated): InvokeFunction used the OpenAPI-generated InvokeFunctionWithResponse client and only treated the call as success when JSONDefault/JSON200 were populated. Those fields are only filled for JSON object bodies. For plain text / empty / non-JSON content-types, the typed fields stay nil and the caller fell through to error classification — producing messages like HTTP 200: plain text from function or HTTP 204: No Content. For application/json arrays (and other non-object JSON), the failure is even earlier: ParseInvokeFunctionClientResponse fails during json.Unmarshal into FunctionInvocationResponse (map[string]interface{}), so InvokeFunctionWithResponse returns a parse error before status-based success handling runs.
• Fix: InvokeFunction calls the raw (non-WithResponse) client method, classifies success purely by HTTP status (2xx = success), and decodes the body leniently (JSON object as-is; other JSON/plain text under a "body" key; empty body as {}). Non-2xx responses still go through apiError normalization. No API request/response contract or endpoint changes; only internal/api/functions.go and its tests.

Validation (independent review)

• Reproduced the pre-fix bug against main for plain-text 200, JSON-array 200 (unmarshal/parse failure), and empty 204; confirmed the CLI path surfaces Error: failed to invoke function "...": HTTP 200: ....
• Confirmed the fix returns success on those same cases (including via the functions invoke command harness).
• Ran full suite: go build ./..., go vet ./..., go test ./... -count=1 — all passed.
• Regression hunt: non-2xx paths still error with status preserved for structured error / error_description / message, 400/401/403/404/429/503, plain-text and HTML error bodies, non-error JSON objects/arrays, empty bodies, malformed JSON, and 302 redirects. No case found where a real failure is reported as success.

Test plan

• Unit: TestInvokeFunctionSuccessWithNonObjectBody (plain text, no content-type object JSON, JSON array, empty 204)
• Unit: existing TestInvokeFunctionErrorsNormalize (429) still passes
• Full go test ./... -count=1
• Optional smoke: deploy/invoke a function that returns plain text or a JSON array and confirm volcano functions invoke prints success JSON

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[CLAassistant]

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