fix(functions): raise FunctionsRelayError for x-relay-error responses

[hsusul]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

Fixes #1637

invoke() is supposed to raise FunctionsRelayError when the Supabase relay cannot reach the Edge Function. Two things prevent that:

1. The client reads x-relay-header. The relay, supabase-js, supabase-swift, and supabase_flutter all use x-relay-error.
2. _request calls raise_for_status() before any relay-header check. Relay failures are non-2xx (supabase-swift#1112), so they become FunctionsHttpError even if the header name were corrected.

The existing tests mock x-relay-header on a 200, so they pass while except FunctionsRelayError is dead in production. The generated sync client has the same code.

What is the new behavior?

_request checks x-relay-error == "true" before raise_for_status() and raises FunctionsRelayError with the response status, matching supabase-js:

const isRelayError = response.headers.get('x-relay-error')
if (isRelayError && isRelayError === 'true') {
  throw new FunctionsRelayError(response)
}

A function-side 4xx/5xx without that header is still FunctionsHttpError. Public invoke() arguments are unchanged.

Open PRs #1576 and #1600 touch nearby error-body parsing in the same files but still read x-relay-header; this change is the wire-format/order fix.

Tests

test_invoke_with_relay_error now sends the real x-relay-error header. New test_invoke_relay_error_on_non_2xx_status returns 546 plus x-relay-error: true and asserts FunctionsRelayError (not FunctionsHttpError). Both fail on main @ bb7ecc5 and pass here, async and sync.

Validation

• Focused tests, red on main (DID NOT RAISE / FunctionsHttpError), green after: 4 passed.
• uv run --package supabase_functions pytest: 67 passed, 1 skipped (test_strenum_import_python_310_and_below on 3.13).
• uv run --package supabase_functions mypy src/supabase_functions tests: no issues in 16 source files.
• uv run ruff check / uv run ruff format --check on src/functions: clean.
• git diff --check: clean.

Functions tests are unit tests with httpx mocks and do not need Docker or the Supabase CLI.