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-publisher.php

@@ -82,6 +82,23 @@ class PRAutoBlogger_Publisher {
 		?string $run_id = null,
 		?PRAutoBlogger_Cost_Tracker $cost_tracker = null
 	): int {
+		// v0.18.1 belt-and-braces: never create a post whose content is
+		// empty once tags and whitespace are stripped — an empty "draft
+		// for human review" (2026-06-11 incident, post 921) reviews
+		// nothing and hides the underlying generation failure. The
+		// provider/writer guards make this unreachable on OpenRouter
+		// paths; this protects every other path into post creation.
+		if ( '' === trim( wp_strip_all_tags( $content ) ) ) {
+			throw new \RuntimeException(
+				sprintf(
+					/* translators: 1: intended post status, 2: article title. */
+					__( 'Refusing to create %1$s post "%2$s": generated content is empty.', 'prautoblogger' ),
+					$post_status,
+					$idea->get_suggested_title()
+				)
+			);
+		}
+
 		// v0.18.0 idempotency: post creation is keyed by run + idea
 		// (check-before-insert) so a retried/resumed run cannot create a
 		// duplicate post. One run_id spans all N articles of a batch, so