fix(thread/github): degrade a non-JSON sandbox-git response instead of raw SyntaxError

[pedrofrxncx]

Source: bug found while auditing the GitHub tooling layer (apps/web/src/components/thread/github/). This is the same failure-mode this repo has already fixed at several other HTTP boundaries (see apps/api/src/tools/github/graphql.ts's parseGraphqlBody, and the merged "degrade a malformed 2xx body instead of raw SyntaxError" lane, e.g. #6289/#6293/#6161) — sandbox-git-api.ts's shared parseJson helper never got the same treatment.

Payoff: every Fast Preview git call (/git/status, /git/diff, /git/publish, /git/rebase, /git/discard, /git/suggest-commit, /git/judge-review) goes through sandboxFetch + parseJson. A gateway/proxy hiccup between the browser and the sandbox daemon (a 502/504 HTML error page, or an empty body on a dropped connection) is not guaranteed to be JSON even when it's a real response, and res.json() throws a raw, unhandled SyntaxError in that case ("Unexpected token '<'") instead of the typed SandboxGitError every caller in this file branches on — including isSandboxUnreachable, which backs off polling instead of hammering a downed sandbox. A raw SyntaxError bypasses that backoff and surfaces as an opaque toast.

Fix: parseJson now reads the body as text and tolerates a JSON-parse failure on both the success and failure path, always throwing SandboxGitError (carrying the real HTTP status) instead of letting JSON.parse's exception escape uncaught. Behavior for a well-formed JSON response (ok or error-shaped) is unchanged.

Regression test: added to the existing sandbox-git-api.test.ts (which already covers this file's pure logic) — a non-JSON error page on a non-2xx status, and a non-JSON body on a 2xx status, both now reject with SandboxGitError carrying the response's status instead of an unhandled SyntaxError.

To verify: bun test apps/web/src/components/thread/github/sandbox-git-api.test.ts

Locally ran: bun run fmt, cd apps/web && bunx tsc --noEmit, the targeted test file above (76 pass), and bunx oxlint on both changed files (0 warnings/errors). Full CI validates the rest.

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Summary by cubic

Degrades non-JSON responses from sandbox git endpoints to a typed SandboxGitError with HTTP status instead of leaking a raw SyntaxError. This restores backoff and consistent error handling across Fast Preview git operations; behavior for valid JSON is unchanged.

• parseJson now reads the body as text, attempts JSON parse, and throws SandboxGitError(status) on parse failure or any non-2xx response.
• Exports SandboxGitError and parseJson; adds tests covering non-JSON bodies (2xx and non-2xx) and JSON error shapes.
• No call-site changes required; callers that branch on SandboxGitError (e.g., isSandboxUnreachable) now back off on gateway errors instead of showing opaque toasts.

^{Written for commit 4c8d5fc. Summary will update on new commits.}