perf(waterdata): compact CQL2 JSON to halve POST chunk count

[thodson-usgs]

Stacked on DOI-USGS#285 (base: httpx-migration).

What

monitoring-locations is the one service that sends its multi-value filter as a CQL2 POST body (it doesn't accept comma-separated multi-value GET params). That body was pretty-printed with json.dumps(..., indent=4) (~39 bytes/value). The body counts against both the server's ~8 KB request-size limit and the chunk planner's byte budget (chunking._request_bytes = URL + body), so the indentation was doubling the per-value cost and therefore doubling the chunk count for large id lists.

This switches _cql2_param to the tightest separators json.dumps(..., separators=(",", ":")) (~17 bytes/value).

Impact

Chunk count (and API requests) for a monitoring-locations id-list query, at the production 8000-byte limit, via the real ChunkPlan planner:

n_ids	indent=4 (old)	compact (new)	reduction
200	1	1	—
500	4	2	2× fewer
1000	8	4	2× fewer
2000	16	8	2× fewer
5000	32	16	2× fewer

Live end-to-end: a 500-id query returns all 500 rows in 2 sub-requests (was 4), no 403.

Safety

Empirically pinned the server's request-size ceilings: GET URLs 414 above ~8.2–8.4 KB; POST bodies 403 above ~8.2–8.4 KB. The chunker's existing 8000-byte limit keeps compact bodies (≤ ~7.75 KB) safely under the 403 cutoff, so no limit change is needed. A compactness assertion on the monitoring-locations POST test locks the format in.

Why not route daily/time-series through POST too?

Investigated and rejected: the server enforces the same ~8 KB cap on total request size whether the bytes are in the URL (414) or the body (403). A compact POST body fits ~450 sites; an 8000-byte GET URL fits ~450 sites — same capacity. So POST can't reduce the chunk count for the GET-based time-series endpoints; this compact win applies only to the pre-existing POST path. The real levers for those (the ~8 KB WAF rule and the 10 K-row page cap) are server-side.

🤖 Generated with Claude Code

[thodson-usgs]

Superseded: reopened against DOI-USGS:main as a standalone PR (the change doesn't depend on DOI-USGS#285).