Reuse template enhancement output buffer in Optimization Detective for WP 6.9

[westonruter]

Instead of Optimization Detective hackily opening its own output buffer via od_buffer_output() during the template_include filter:

performance/plugins/optimization-detective/hooks.php

Line 19 in a5c444f

add_filter( 'template_include', 'od_buffer_output', PHP_INT_MAX );

In WordPress 6.9 we can now leverage the template enhancement output buffer introduced via Core-43258 (see r60936). When the wp_start_template_enhancement_output_buffer() function exists, we can skip starting our own output buffer:

if ( ! function_exists( 'wp_start_template_enhancement_output_buffer' ) ) {
    add_filter( 'template_include', 'od_buffer_output', PHP_INT_MAX );
}

And then od_template_output_buffer() can be updated accordingly:

if ( function_exists( 'wp_start_template_enhancement_output_buffer' ) ) {
    add_filter( 'wp_template_enhancement_output_buffer', $callback );
} else {
    add_filter( 'od_template_output_buffer', $callback );
}

[westonruter]

Depends on #2354

[MarcinDudekDev]

I've opened #2516 to migrate Optimization Detective to the WordPress 6.9 wp_template_enhancement_output_buffer API for this.

Heads-up that #2514 by @Hug0-Drelon (opened a little earlier today) also addresses this issue — maintainers will likely want to pick one. The two take slightly different approaches:

• Use wp_template_enhancement_output_buffer hook when available. #2514 uses the new hook "when available", keeping the existing od_buffer_output path as a fallback for WordPress < 6.9.
• Optimization Detective: Reuse the WordPress 6.9 template enhancement output buffer #2516 assumes the 6.9 minimum (per Bump minimum WordPress version to at least 6.7 (Yes, “SIX SEVEN”) but ideally 6.9 #2354), so it drops the function_exists() guard and removes od_buffer_output() entirely. It also adds a backward-compatibility shim (apply_filters_deprecated()) for the now-removed public od_template_output_buffer filter, and updates the docs/tests accordingly.

Happy to defer to #2514 or adapt mine depending on whether the team wants to wait for the 6.9 floor (#2354). Full CI is green on #2516.