fix(responses): preserve whitespace in tool-call arguments (#244)

[ltmoerdani]

Closes #244

Problem

All whitespace is stripped from tool calls and responses on OpenAI models (both OpenCode Go and Zen) — every word concatenated (thisiswhattheresponselookslike), and corrupted tool-call arguments send the model into reasoning loops. Text-only conversations are also affected because agent-mode output flows through tool calls.

Root cause

Regression class of #192 (fixed in PR #194). That fix moved text/reasoning deltas to firstStringRaw() but deliberately left arguments_delta on the trimming firstString() under the assumption that arguments tolerate whitespace stripping.

That assumption was wrong. Responses API argument fragments are arbitrary JSON slices that can split inside string values ("hello wo + rld"), so per-fragment .trim() destroyed boundary whitespace and corrupted tool-call input on every GPT-family model (all route to /v1/responses).

Verified against official sources:

• openai-node's response-accumulator.ts accumulates via raw output.arguments += event.delta — no trim, no falsy filtering
• openai-node's own test mocks split argument streams on whitespace boundaries — exactly the case trimming destroys
• response.function_call_arguments.done carries the final authoritative arguments string

Reporter diagnostics confirmed every affected request hit the responses endpoint (HTTP 200, silent corruption) with the repeat-request/cache-hit pattern of an agent retrying mangled tool calls.

Fix (3 layers)

Layer	File	Change
Root cause	src/core/routing.ts	function_call_arguments.delta → firstStringRaw() (no trim)
Repair net	src/core/routing.ts	New function_call_arguments.done handler → emits final arguments tagged argumentsDone: true
Self-healing	src/toolCallAccumulator.ts	argumentsDone delta REPLACES pending arguments instead of appending — heals any gateway-side delta mis-join

Tests

5 new regression tests: fragment split mid-string-value, done-event mapping, done-without-args, replace semantics, append-only path unaffected. 489/489 pass, npm run compile + typecheck clean.

Docs

• docs/issues/107-20260924-issue244-tool-call-arguments-whitespace.md (new)
• docs/issues/83 annotated — superseded assumption noted
• CHANGELOG [0.7.7], devlog, ARCHITECTURE-MAP
• Version bump 0.7.7 (VSIX built + verified locally)

Notes for merge

Per repo policy: merge commit (gh pr merge --merge), no squash.