fix(prompt): support Anthropic anyOf tool schemas

[kyboi]

Anthropic request construction rejects non-null ToolParameterType.AnyOf parameters before sending a request. Serialise those alternatives as JSON Schema anyOf, preserving nested schemas and non-blank descriptions. Existing nullable-union serialisation remains unchanged.

Regression coverage includes scalar alternatives, a number-or-object parameter, nested unions, blank descriptions and nullable-union compatibility. The three non-null union regressions fail on upstream before the fix.

Validation: all 49 Anthropic JVM tests pass; the JVM JAR build and commonMain and jvmTest ktlint checks pass. Non-JVM targets and live provider calls were not tested. No public API changes.

Adapted from Kroog commit 967531a8c, with additional focused regression coverage.

Fixes #2260.