fix(windows): json unmarshal errors in telemetry and pg-delta declarative sync

[avallete]

Summary

Three Windows-only failures, all surfacing as JSON parse errors. None reproduce on macOS or Linux.

Reported errors

1. supabase telemetry disable

failed to parse telemetry file: Time.UnmarshalJSON: input is not a JSON string

The on-disk ~/.supabase/telemetry.json had session_last_active stored as a number (epoch millis) instead of an RFC3339 string — time.Time.UnmarshalJSON rejects numbers. LoadOrCreateState only handled os.ErrNotExist, so the parse error propagated and the command crashed.

2. supabase db schema declarative sync --experimental (export step)

error exporting declarative schema: edge-runtime script produced no output:
NotFound: No such file or directory (os error 2): readfile
  '/workspace/supabase\.temp\pgdelta\catalog-baseline-17.6.1.106.json'

The catalog file path was constructed on Windows with filepath.Join, which produces \ separators. containerRef prefixed it with /workspace/ and passed it to the Linux edge-runtime container, which cannot read backslash paths. Before this PR the subprocess crashed silently and the user only saw the much less helpful unexpected end of JSON input from json.Unmarshal on empty stdout — surfacing stderr is what made the underlying path bug visible in the first place.

3. supabase db schema declarative sync --experimental (diff step)

error diffing schema: edge-runtime script produced no output:
main worker has been destroyed

For the export and catalog scripts, empty stdout means the subprocess crashed. For the diff script, empty stdout means there are simply no schema changes. The empty-stdout guard therefore lives in the export and catalog callers (which require non-empty output), and the diff caller correctly accepts empty stdout as "no changes".

Fixes

• internal/telemetry/state.go: sentinel errMalformedState so any unmarshal failure (*json.SyntaxError, *json.UnmarshalTypeError, time.Time.UnmarshalJSON, …) recreates state. Identity fields are not worth surfacing an error for; losing them is harmless.
• internal/db/diff/pgdelta.go: containerRef runs filepath.ToSlash so Windows paths normalise before being passed into the Linux container. DeclarativeExportPgDeltaRef and ExportCatalogPgDelta now error with stderr when stdout is empty, instead of failing later with unexpected end of JSON input.
• internal/db/pgcache/cache.go: same empty-stdout guard on exportCatalog.

Test plan

• go build ./...
• TestLoadOrCreateState/recovers_from_corrupted_state_file — table covering empty file, truncated JSON, session_last_active as number, session_last_active as malformed string
• TestContainerRef — Windows path with \ separators, Unix path passthrough, postgres URL passthrough
• Manual on Windows 11: go run . telemetry disable against the actually-corrupted file → succeeds, file rewritten with valid JSON
• Manual on Windows 11: go run . db schema declarative sync --experimental --yes → No schema changes found (previously crashed with unexpected end of JSON input)

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