test: SSE coverage for empty and chunked tool arguments

[yuanhao]

Closes #162. Four tests on the layer that shipped a bug.

Anthropic: the missing sibling

malformed_tool_arguments_fail_the_turn_instead_of_defaulting was already pinned. The empty-argument case was not — and that asymmetry is exactly how the bug escaped: a zero-parameter tool streams input_json_delta carrying "", from_str("") fails with "EOF while parsing a value", and empty got treated as malformed.

Reverting the 0.18.1 fix reproduces the production error byte-for-byte:

tool call(s) with unusable arguments, not executed: `list_files` ()

0.18.1 unit-tested resolve_tool_arguments, but that's the pure decision. This drives an empty stream through the actual parser.

openai_compat: 3 tests, none touching tool calls

For a module backing OpenAI, Groq, Together, DeepSeek, Fireworks, Mistral, xAI. Added three: arguments reassembling across chunks, a zero-argument call, and truncated arguments.

A provider divergence, documented rather than fixed

The truncated-arguments test found something worth knowing:

	on unparseable tool arguments
Anthropic	fails the turn — "a tool handed {"__partial_json": ...} runs on its defaults instead of what the model asked for"
openai_compat	falls back to {} and warns — so the tool does run on its defaults

That's the exact hazard Anthropic's path guards against, happening by default on 15+ providers.

I pinned it rather than changed it. Closing the gap is a behavioural decision with a wide blast radius, not a test fix — and it should be made deliberately, not as a side effect of adding coverage. If that assertion ever changes, it will be on purpose.

Verification

mutation	result
revert the 0.18.1 empty-args fix	reproduces the production error exactly
take only the first argument chunk	arguments split across chunks must reassemble
make the parse fallback fail	catches the divergence test

cargo test --all-features: 633 passed, 0 failed. Clippy clean under -Dwarnings.

Tests only — no library change.

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