IIS: duplicate <preConditions> singleton collision in WebP/AVIF rewrite-rules classes

[Honemo]

🤖 AI-generated — created by an automated pipeline. Review before acting on this.

Source: Follow-up from grooming of issue #509 (COULD_HAVE)

Context
While addressing the critical bug in #509 (duplicate <staticContent> blocks in IIS web.config causing site-wide 500 errors), a related architectural pattern was identified in the sibling rewrite-rules producer classes.

The classes/Webp/RewriteRules/IIS.php and classes/Avif/RewriteRules/IIS.php classes use the same underlying architecture via classes/WriteFile/AbstractIISDirConfFile.php, which exhibits the same root-cause issue: the get_node()/prepend_node() methods perform blind-prepend operations without deduplication logic.

This can produce duplicate <preConditions> singleton node collisions in generated IIS rewrite rules. While #509 was kept focused on the <staticContent> fix, this issue was explicitly scoped out per user decision during grooming.

Suggestion
Apply the same fix pattern from #509 to the rewrite-rules classes. Reference the root-cause analysis in .ai/issues/509/spec.md for AbstractIISDirConfFile.php behavior details.

Acceptance Criteria

• classes/Webp/RewriteRules/IIS.php generates unique <preConditions> nodes without duplicates
• classes/Avif/RewriteRules/IIS.php generates unique <preConditions> nodes without duplicates
• AbstractIISDirConfFile.php deduplication behavior is aligned with Check if rules already exist in IIS (Windows) web.config file #509 fix strategy

Related Issues

• Check if rules already exist in IIS (Windows) web.config file #509: Critical IIS web.config duplicate <staticContent> fix