test: cover wttrin-geolocation internal helpers

Four pure helpers in wttrin-geolocation.el were exercised only indirectly through the parser tests: --decode-json, --format-city-region, --lookup-provider, and --extract-body. None of them had direct unit coverage. Edge cases like an empty JSON object, a missing-vs-empty city field, an unknown provider symbol, or a missing HTTP body separator weren't locked.

The new file groups all four functions together. Each gets Normal, Boundary, and Error cases per testing.md. Highlights:

- --decode-json: distinguishes nil input, empty string, and malformed JSON, all of which return nil for different reasons.
- --format-city-region: separates "missing key" from "empty string" since the predicate `(and (stringp city) (> (length city) 0) ...)` short-circuits on either.
- --lookup-provider: tests two of the three built-ins plus a `let`-bound synthetic provider, locking the documented extension point on `wttrin-geolocation--providers`.
- --extract-body: real UTF-8 bytes inserted into a temp buffer (mirroring what `url-retrieve` delivers) verify the decoding path. 4xx, 5xx, and missing-separator paths each get their own test.

21 new tests, all green on first run since they characterize existing behavior.

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