fix(db export): explain unsupported Fly exports

[ada-powerful]

Fixes #981.

Problem

When turso db export is run against a database whose export endpoints are unavailable, the CLI currently bubbles up a raw HTTP error like:

failed to export database: response failed with status 404 Not Found

For Fly-hosted databases, that does not explain what the user can do next.

Fix

Handle 404 responses in the export path with a specific message explaining that database export is unavailable for that database and pointing users to turso db shell <database> .dump as the SQL dump fallback. Other export errors still use the existing response parser.

Added a regression test for the /info 404 path so the raw response failed with status 404 Not Found message does not come back.

Tests

docker run --rm -v "$PWD":/src -w /src golang:1.25 gofmt -w internal/turso/tursoServer.go internal/turso/tursoServer_test.go
docker run --rm -v "$PWD":/src -w /src golang:1.25 go test ./internal/turso -run "TestExportReportsUnsupportedDatabaseForNotFound|TestUploadFileMultipart_ContentLengthHeader"
docker run --rm -v "$PWD":/src -w /src golang:1.25 go test ./internal/turso

Disclosure

This PR was prepared by an AI agent under direct human review. Happy to adjust the wording if there is a preferred export fallback message.