fix(openai): omit absent tool description, never serialise a null schema

[AlexMikhalev]

The OpenAI adapter built each tool with json!, so both optional fields serialised as explicit nulls when unset:

"description": tool.description   // None -> null
"parameters":  tool.schema        // None -> null

Neither is valid. description is optional in the OpenAI function schema, and omitting a key is not the same as sending null — providers that validate the payload reject a null where a string is expected. parameters is worse: it is required to be a schema object, so null is not a legal value at all.

Builds the function object explicitly instead: description is inserted only when present, and parameters falls back to {"type": "object"} — the empty schema, which is what a tool taking no arguments actually means.

strict: false is deliberately unchanged. It is the documented default, and altering it would change structured-output behaviour for every adapter sharing this serialiser. The existing TODO about strict: true needing additionalProperties: false is left for whoever takes that on.

Why it is reachable

terraphim-llm-proxy accepts tools in both the OpenAI shape ({type, function:{...}}) and the Anthropic shape ({name, input_schema}), and the Anthropic form frequently carries no description. It began routing tools through this serialiser in terraphim-llm-proxy#89.

Tests

Five added; the first and third fail against the previous code.

• absent description omitted rather than nulled
• present description still serialises
• absent schema falls back to the empty object schema, never null
• present schema passes through verbatim
• strict and the type: function envelope preserved

cargo test --lib: 70 passed. cargo fmt --check: clean.

Pre-existing, untouched: cargo clippy -- -D warnings fails on src/webc/web_stream.rs (collapsible_match) on main as well.