fermi: LATDataQuery.cgi is being replaced by a REST API — module update available

## Summary
I work on the Fermi LAT data-server infrastructure at NASA/GSFC. The
`LATDataQuery.cgi` endpoint that `astroquery.fermi` submits queries to is
being replaced by a JSON REST API, now live at:

- Docs: https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lat_data_query_doc.html
- Base URL: `https://fermi.gsfc.nasa.gov/ssc/data/access/lat/query/api/v1`

## New API in brief
```
POST /query -> {"query_id": ...} (JSON payload)
GET /query/{id}/status -> {"state": ...}
GET /query/{id}/results -> {"files": [{"name": ...}, ...]}
```
Files are staged on the HEASARC FTP area as before. Most request field names
were deliberately kept compatible with the CGI form fields
(`coordfield`, `coordsystem`, `shapefield`, `timefield`, `timetype`, `energyfield`,
`photonOrExtendedOrNone`, `spacecraft`), and a
new optional `zenithangle` parameter was added. `coordfield` accepts either
`"RA,Dec"` or a target name, `coordsystem` accepts `J2000`, `B1950` or
`Galactic`, and all-sky queries are supported with a radius > 60 degrees and
an observation window of <= 24 hours.

## What this means for astroquery
I have a rewritten `astroquery/fermi` module ready as a PR:

- **`FermiLAT.query_object()` keeps its existing signature** — end-user
scripts should not need to change.
- All HTML scraping is removed; the module speaks JSON only.
- `query_object_async()` now returns the server's `query_id` instead of a
results-page URL.
- Invalid queries are rejected with a JSON error body `{"error": "..."}`, which the module now surfaces to users — this addresses the long-open #1849.
- New capabilities exposed: `get_status()`, `list_results()`,
`wait_for_completion()`, `get_file_urls()`, plus `zenithangle` and all-sky queries.
- `GetFermilatDatafile` / `get_fermilat_datafile` retained as deprecated
shims.
- Unit tests rewritten against mocked JSON responses; remote tests verified
end-to-end against the live API (all passing as of 2026-08-14).

I'll open the PR shortly and link it here. Feedback welcome on the approach —
in particular on the deprecation handling and on whether the module's
"Experimental" import warning should be dropped as part of this change (the
PR currently drops it).

Start with the astroquery/fermi module and its rewritten unit tests with mocked JSON responses, then compare the implementation with the documented REST endpoints. Done means preserving query_object() compatibility, exposing the listed status and result capabilities, surfacing JSON errors, retaining deprecated shims, and passing the mocked and remote tests described in the issue.

Written by the indexing model from the issue text.