[Image] Stop suggesting image_id for engines that drop it

[ilyazub]

Opus 5.5 Max fix, tests and checks:

Fixes --image's advice before v0.4.0 ships. The error told agents to upload with serpapi image and pass image_id=<id> to another engine. SerpApi takes image_id only on google_lens, though: other engines drop it and run a normal, counted search without the image. Follow-up of #14.

This is the release PR for #16 (--debug) and #14 (image). Both merged with release:skip, and this one carries release:minor, so merging it releases v0.4.0 with all three.

On main 433aa35 → this PR:

search … --image photo.png	main 433aa35	this PR
engine=google q=cat	exit 2, "…for another engine, upload with 'serpapi image FILE' and pass image_id="	exit 2, "…other engines drop image_id and search without the image"
engine=Google_Lens (SerpApi lowercases it)	exit 2, refused	exit 0, upload, then search
engine=google_lens url=… (Lens refuses url next to image_id)	exit 0 here; uploads, then searches	exit 2 before the upload

The fake-API tests behind the table fail on 433aa35: 4 of the 5 refusal cases plus the Google_Lens test, all passing here.

Docs:

• The image help, the Cost line and the README say an upload doesn't count against your searches, but it needs searches left this month and this hour, and has its own hourly limit (account_rate_limit_per_hour).
• The README's Lens example used type=products, which SerpApi rejects as discontinued; it now uses type=exact_matches.

Checks on 5a768a1, no API key and a dead proxy, so nothing reached SerpApi:

gofmt, go vet, staticcheck (go1.25.13)                exit 0
go test -race ./... on go1.27.2 and go1.25.13          exit 0, 8/8 packages each
builds: linux, darwin (amd64, arm64), windows/amd64    exit 0

Not covered: the live e2e tests, which run in CI.