fix: recover reasoning-only compact summaries instead of failing with EmptyResponse

[minhtq1234]

Problem

Running /compact (and autocompact) fails with Compact failed: Empty response from LLM when the conversation model is a reasoning model served via an OpenAI‑compatible endpoint. Observed in AionUi with MiniMax M2.5 (custom provider): the model generates for ~17s, then the turn ends with event_type="Error" text_len=0.

Root cause

• The compact request sets thinking: ThinkingConfig::Disabled (crates/aion-agent/src/compact/auto.rs), but the OpenAI projector only acts on ThinkingConfig::Enabled (crates/aion-providers/src/projector.rs), so "disabled" never reaches the wire — reasoning models reason anyway.
• The OpenAI-compatible stream parser maps reasoning_content deltas to LlmEvent::ThinkingDelta (crates/aion-providers/src/openai.rs).
• collect_stream_text in compact/auto.rs accumulated only TextDelta and discarded thinking deltas. A model that emits its whole summary as reasoning yields an empty string → CompactError::EmptyResponse, even though a complete summary was produced (output budget is not the issue — COMPACT_MAX_OUTPUT_TOKENS is 20k).

Fix

collect_stream_text now accumulates thinking deltas alongside text and falls back to the reasoning content when the model produced no plain text. format_compact_summary already degrades gracefully when <summary> tags are absent, so the recovered content flows through the existing parsing unchanged.

Behavior is otherwise preserved:

• Text output, when present, always wins — reasoning is only used as a fallback.
• A truly empty response (no text, no reasoning) still fails with EmptyResponse.
• A channel that closes without Done can now salvage reasoning-only output; if nothing was received it still fails with EmptyResponse as before.

Testing

• New regression test reasoning_only_response_is_recovered (ThinkingDelta-only stream → compaction succeeds, summary contains the recovered content, failure counter untouched).
• Existing empty_response_fails still passes (truly empty stream → EmptyResponse).
• cargo test -p aion-agent — all tests pass.
• cargo fmt --all -- --check and cargo clippy -p aion-agent -- -D warnings — clean.

Repro context

AionUi desktop → aionrs conversation → MiniMax M2.5 (custom OpenAI-compatible provider) → /compact → Compact failed: Empty response from LLM. With this change the reasoning-only summary is recovered and compaction completes.