chore: scrub stale filter argument references in comments/errors

[pyramation]

Summary

Follow-up cleanup to #995. The GraphQL connection argument was renamed from filter to where in commit 10b4fc6 (via connectionFilterArgumentName: 'where' defaulted in graphile-connection-filter's preset), but a number of JSDoc examples, plugin descriptions, error strings, and preset comments were not updated at the time. This PR scrubs them.

Changes are almost entirely in comments/docstrings. The only behavior-adjacent diffs are:

1. Two error message strings renamed:
   • "Empty objects are forbidden in filter argument input." → "...in where argument input."
   • "Null literals are forbidden in filter argument input." → "...in where argument input."
2. One test case title renamed to match what it actually asserts (where arg, not filter).
3. One internal extend(...) log string renamed (not user-visible, just debug output).

Specifically updated:

• graphile-connection-filter: JSDoc examples (filter: → where:) in ConnectionFilter{Forward,Backward,ComputedAttributes}RelationsPlugin, the ConnectionFilterArgPlugin top-level JSDoc + description string + inline comments, and the two error message strings in utils.ts, ConnectionFilterAttributesPlugin.ts, and operatorApply.ts.
• graphile-postgis: JSDoc example in spatial-relations.ts.
• graphile-settings: corrected the misleading preset comments in constructive-preset.ts (the ones that tripped me up in docs(graphile-postgis): deep spatial-relations docs; lead with client-side GeoJSON problem #995 and made me think the GraphQL arg was still filter:), updated an example comment in enable-all-filter-columns.ts, and renamed the preset-integration test case title.

Review & Testing Checklist for Human

• Confirm you're OK with the two error message text changes — anyone downstream matching on the exact string '... forbidden in filter argument input.' (equality check, not substring) would need to update their match. I grepped the repo for these strings and the only call sites are the three source files in this diff; no test snapshots reference them.
• Skim the preset comments in graphile/graphile-settings/src/presets/constructive-preset.ts — the rewritten block about "filtering now lives under the where argument" should read accurately to you.

Notes

• No runtime behavior change other than the two error message strings mentioned above.
• Ran the individual graphile-connection-filter and graphile-postgis package builds locally; both clean. The workspace-level build fails on unrelated missing cross-package type declarations in my environment, but CI will do the full build.
• CHANGELOG entries that reference the historical filter → where rename commit were deliberately left alone — they're historical and accurate.

Link to Devin session: https://app.devin.ai/sessions/059b0c2e4594474aa107e3f83f83fe0f
Requested by: @pyramation

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