Simplify AppBuilder tool initialisation (#47)

[leynos]

Summary

This branch simplifies AppBuilder::init_tools for issue #47 by moving media tool credential and model resolution into a private helper, replacing duplicate image and vision registration wiring with the shared bootstrap helper, and naming the builder-tool registration predicate.

Closes #47.

Review walkthrough

• Start with src/app.rs to review the extracted resolve_media_tools_config and should_register_builder_tool helpers.
• Then review the updated AppBuilder::init_tools body in src/app.rs to confirm the media and builder registration flow remains behaviour-preserving.

Validation

• coderabbit review --agent: passed with 0 findings.
• make all: passed format checks, clippy, GitHub WASM tool build, 4,146 nextest tests, and 5 GitHub tool tests.

Notes

No public APIs changed. The branch deliberately keeps the refactor inside the application composition root and reuses the existing MediaToolsArgs and register_image_and_vision_tools bootstrap API.

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