feat: make integration proxy paths configurable per customer

[vasujain00]

Fixes #741

Summary

• Adds a proxy_prefix() method to the IntegrationProxy trait allowing integrations to use customer-configured paths instead of hardcoded /integrations/{name} paths
• Implements configurable proxy_path for the Didomi integration as the first adopter
• Backwards-compatible: default behavior unchanged when proxy_path is not set

Changes

File	Change
crates/trusted-server-core/src/integrations/registry.rs	Added proxy_prefix() method to IntegrationProxy trait; updated get(), post(), put(), delete(), patch() helpers to use it
crates/trusted-server-core/src/integrations/didomi.rs	Added proxy_path config field; overrides proxy_prefix() when set; updated routes and path stripping

Closes

Closes #741

Test plan

• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: New test registers_custom_proxy_path verifies custom path routes correctly and default path no longer matches

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code -- use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed

[ChristianPavilonis]

Thanks for the contribution — making these proxy paths configurable is a good direction, and I appreciate that the default route behavior is covered. I found two implementation gaps that look worth addressing before relying on custom Didomi paths.

I left the main items inline. One additional follow-up: once the behavior is wired through, please update docs/guide/integrations/didomi.md to document proxy_path, its default, accepted format, and the custom endpoint examples.