Fix render debug overlay panic without bevy_pbr (#25980)

[jorgeandrecastro]

Objective
Fixes #25980

• Stops a startup crash in Bevy apps that turn on the dev feature (which brings in RenderDebugOverlayPlugin) without having the bevy_pbr feature around.

Solution

• I Made the MeshPipelineViewLayouts resource optional in RenderDebugOverlayPlugin by wrapping it in Option<Res> inside crates/bevy_dev_tools/src/render_debug.rs.

• An i added safe checks for when it's missing during pipeline initialization and specialization, so non-PBR or pure 2D setups won't panic anymore.

Testing

• Did you test these changes? Yes, verified by running the 2d_shapes example with the dev feature enabled.

• Are there any parts that need more testing? Nope.

• How can other people test your changes? Just run cargo run --example 2d_shapes --features dev and watch it boot up cleanly without throwing a missing resource erro

• What platforms did you test on? Tested on Linux (Debian 13, Vulkan with Intel Iris Xe Graphics).

Showcase
The 2d_shapes example now starts up and runs without issues with dev features:

cargo run --example 2d_shapes --features dev

Thanks for reading :)

[mockersf]

could you remove most of the comments to keep to the existing style? And only the first one is useful

[jorgeandrecastro]

could you remove most of the comments to keep to the existing style? And only the first one is useful

Done! I cleaned up the extra comments and kept just the first one pointing out that it's optional. Thanks for the guidance! :)