fix(worker): treat empty LLM response after text output as completion (#1677)

* fix(worker): treat empty LLM response after text output as completion

When a job's LLM produces a substantive text response (e.g., formatted
results from a routine) and the next LLM call returns empty or errors,
the worker now treats this as successful completion instead of
continuing the loop until failure.

Previously, empty responses always triggered TextAction::Continue,
causing the loop to re-call the LLM. The LLM had nothing more to say,
so the provider returned "Response contained no message or tool call
(empty)". This made routine jobs that successfully produced results
report as "failed".

The fix adds a `has_text_response` flag to JobDelegate:
- After any non-empty text response: flag is set
- Empty text after flag is set: treated as completion
- LLM errors (select_tools/respond_with_tools) after flag: treated
  as completion instead of propagating
- Empty text before any output: still retries (rate-limit backoff)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix(worker): restrict error swallowing to EmptyResponse variant only

- Add LlmError::EmptyResponse variant for when LLM returns no content
- Update nearai_chat and github_copilot providers to emit EmptyResponse
  instead of InvalidResponse for empty/no-choice responses
- try_complete_on_error now only swallows EmptyResponse (not AuthFailed,
  ContextLengthExceeded, Http, Io, etc.)
- Extract is_completion_eligible_error as testable pure function
- Log mark_completed errors at warn level instead of silently dropping
- Add EmptyResponse to retry and circuit breaker transient classifications
- Rewrite test to exercise real classification logic against all variants

Addresses review feedback from zmanian and gemini-code-assist.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* refactor(worker): extract mark_completed_or_warn helper to DRY completion logic

Extract shared mark-completed + warn-on-failure pattern into a single
helper method used by both try_complete_on_error and handle_text_response.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: j-bloggs <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/llm/github_copilot.rs

@@ -231,9 +231,8 @@ impl LlmProvider for GithubCopilotProvider {
                 .choices
                 .into_iter()
                 .next()
-                .ok_or_else(|| LlmError::InvalidResponse {
+                .ok_or_else(|| LlmError::EmptyResponse {
                     provider: "github_copilot".to_string(),
-                    reason: "No choices in response".to_string(),
                 })?;
 
         let (content, _tool_calls) = extract_choice_content(&choice);
@@ -309,9 +308,8 @@ impl LlmProvider for GithubCopilotProvider {
                 .choices
                 .into_iter()
                 .next()
-                .ok_or_else(|| LlmError::InvalidResponse {
+                .ok_or_else(|| LlmError::EmptyResponse {
                     provider: "github_copilot".to_string(),
-                    reason: "No choices in response".to_string(),
                 })?;
 
         let (content, tool_calls) = extract_choice_content(&choice);