fix: empty-content guard + finish_reason=length handling + reasoning budget (v0.18.1)

Reasoning models at high effort could spend the entire max_tokens budget
on thinking and return zero visible content with finish_reason=length;
the empty article flowed writer -> editor (reject) -> empty draft post,
with the cost booked as success (2026-06-11 incident: prod run acf24029,
draft post 921; diagnosis: convo pipeline-v2-phase1 seq 12).

- Empty-completion guard at the provider seam (new
  OpenRouter_Completion_Guard): empty/whitespace content is a failure;
  one retry with reasoning disabled (per-call override) when
  finish_reason=length or reasoning was active, then the stage fails.
- finish_reason=length always warning-logged (model, stage, tokens).
- Failed/retried attempts booked to generation_log with
  response_status=error + real token burn; run_decisions verdict rows
  (retry_reasoning_disabled / failed_empty_completion) for v0.18.0 runs.
- Writer-path guard in Content_Generator::execute_stage and
  belt-and-braces empty check in Publisher::create_post.
- Reasoning budget sanity: reasoning.max_tokens cap + completion
  headroom via new prautoblogger_reasoning_max_tokens setting (default
  constant 2048, 0 = legacy uncapped effort mode); cap replaces effort
  when active; governor reservations use the effective ceiling.
- Body assembly moved to Request_Builder::build_body (300-line cap).
- Tests: guard verdict matrix, builder budget shaping, writer/publisher
  guards, full provider retry path.

Agent-Session: prab-empty-draft-guard-20260611

includes/core/class-content-generator.php

@@ -197,6 +197,12 @@ class PRAutoBlogger_Content_Generator {
 			PRAutoBlogger_Run_Stage_State::start( $this->run_id, $stage, '', $this->item_key );
 		}
 
+		// v0.18.1: stage metadata for the provider's empty-completion
+		// guard — warning logs and failed-attempt audit rows carry the
+		// real stage + registry key (never sent upstream).
+		$options['stage']      = $stage;
+		$options['prompt_key'] = $prompt_key;
+
 		$ctx = PRAutoBlogger_Opik_Trace_Context::current();
 
 		$span_id = $ctx->start_span(
@@ -223,6 +229,21 @@ class PRAutoBlogger_Content_Generator {
 			$options
 		);
 
+		// v0.18.1 writer-path guard (belt over the provider seam, which
+		// already throws for OpenRouter): no empty article may ever leave
+		// a writing stage — it would flow to the editor, be rejected as
+		// "draft text is missing", and land as an empty draft post.
+		if ( '' === trim( (string) $response['content'] ) ) {
+			throw new \RuntimeException(
+				sprintf(
+					/* translators: 1: writing stage name, 2: model identifier. */
+					__( 'Writing stage "%1$s" produced empty content (model %2$s) — failing the stage instead of passing an empty article downstream.', 'prautoblogger' ),
+					$stage,
+					(string) $response['model']
+				)
+			);
+		}
+
 		$ctx->end_span(
 			$span_id,
 			array(