fix(gfql): preflight remote engine requests - #2005

Summary

• share remote-engine preflight between gfql_remote() and python_remote()
• preserve explicit pandas/cuDF requests unchanged in the request body
• reject unsupported explicit engines with typed E405 before auth refresh, upload, or POST, with an accurate engine-specific diagnostic

This completes the remaining #1957 explicit-engine honesty and side-effect contract. The Polars work in this release is the local GFQL polars/polars-gpu engine; the public remote contract remains pandas/cuDF. Unsupported Polars-backed remote frames already decline before transport, and this PR makes explicit unsupported engine requests decline consistently. engine="auto" remains unchanged.

Exact state

• base: master at a2b1b44662161ed886665ddf92f7b14fd53b1bda
• head: 55161a11d57207a66dd977279c5cccabddfb84e0
• scope: 8 files, +205/-36
• GitHub: MERGEABLE; BLOCKED reflects required review only
• all 3 review threads are resolved and current

Validation

• hosted exact-head rollup: 61 passed, 7 conditional skips, 0 failed/cancelled, 0 pending
• Read the Docs and downstream changed-line coverage passed
• bounded local focused suite: 72 passed, 2 expected optional skips
• Ruff passed all 7 changed Python files
• touched-file mypy passed all 7 changed Python files
• type-hygiene and comment-density ratchets show no growth
• complete added-line prohibited-pattern scan has zero hits
• git diff --check is clean
• no live remote service, Docker, local GPU, DGX, or broad suite was used

Reviewer decisions

1. Confirm both public remote APIs resolve unsupported explicit engines before auth refresh, upload, or POST.
2. Confirm explicit pandas and cuDF values reach the request body unchanged.
3. Confirm AUTO behavior is intentionally unchanged.
4. Confirm E405 carries accurate API, field, value, and suggestion context.
5. Confirm local Polars support does not imply a remote Polars execution protocol in this release.

Landing

PR #2004 is already merged and this PR is rebased directly onto current master; there is no dependent successor. The branch-development agent will not merge this PR without separate owner authorization.

Closes #1957