Rewrite TESTCONTAINERS-USAGE.md against the current code #168

Problem

TESTCONTAINERS-USAGE.md is the most stale document in the tree. It describes an earlier checkout:

• File paths point at /home/magnus/src/local/turso-java/..., a directory that no longer exists.
• Packages org.libsql.jdbc.base and org.libsql.jdbc.integration; the real classes live in com.github.magnusp.libsql.itest (SqldContainerBase, NetworkToxicityIT, HikariCpIntegrationIT, and the other *IT classes).
• The example class name NetworkToxicityIntegrationIT does not exist; the class is NetworkToxicityIT.

Note on endpoints. The test classes probe /v2/pipeline for readiness (SqldContainerBase, NetworkToxicityIT, NamespacePathIT, and XerialTestServer), while the driver itself posts Hrana 3 pipeline requests to /v3/pipeline (LibsqlHttpClient). The guide must state both faithfully. /v2/pipeline in the readiness probe is current, not stale.

Proposed change

Rewrite the guide against the actual sources under libsql-itests/src/test/java/com/github/magnusp/libsql/itest/, referencing:

• The pinned image via the root pom.xml libsql.image property (already correct).
• The DOCKER_CONFIG workaround from AGENTS.md §4 rather than any hard-coded host path.
• The real SqldContainerBase, NetworkToxicityIT, and HikariCpIntegrationIT classes and their packages.

Transcribe what the tests actually do; do not invent new setup code.

Acceptance criteria

• No /home/magnus/src/local/turso-java or org.libsql references remain.
• The readiness-probe snippets keep /v2/pipeline to match the real test classes, and the driver endpoint is described as /v3/pipeline.
• The guide's code snippets match the real test classes.