fix: handle empty/error bodies in TokenTester token exchange

[antosubash]

Summary

Fixes the cryptic error Token exchange failed: Failed to execute 'json' on 'Response': Unexpected end of JSON input shown by the dashboard TokenTester after an OAuth callback.

Root cause

In modules/Dashboard/src/SimpleModule.Dashboard/Pages/components/TokenTester.tsx, the /connect/token response was parsed via await res.json() unconditionally. Two failure modes:

1. Empty body (e.g. some non-2xx responses with no body) — res.json() itself throws Unexpected end of JSON input, which becomes the user-visible error.
2. OpenIddict error JSON (400 with { error, error_description }) — the !res.ok branch threw ${res.status} ${res.statusText} and discarded the body, hiding the actual reason for the failure (e.g. invalid_grant, PKCE mismatch, etc.).

Change

• Read the response body as text once, then attempt JSON.parse only if non-empty.
• On !res.ok, surface error_description / error from the parsed body when available.
• On success, validate that access_token is present before using it.
• Add Accept: application/json to be explicit about the expected response type.

Test plan

• Trigger the OAuth flow from the Dashboard "Get Token" button and confirm the access token decodes successfully.
• Force an error (e.g. tamper with the state or code_verifier) and confirm the alert now shows the OpenIddict error_description instead of the generic JSON-parse error.

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