Improve diagnostics for overlong asset paths

[aiqubits]

Summary

• guard ConstStr capacity before writing into its fixed-size buffer
• report an actionable compile-time error when a resolved asset path exceeds the 256-byte limit
• validate CLI-created source and bundled paths, including hash-generated and public-directory assets
• propagate path-length errors before modifying the output binary
• include the offending path and original source in runtime diagnostics
• add boundary, overflow, diagnostic, and compile-fail coverage

Testing

• cargo test --offline -p const-serialize -p manganis-core -p manganis-macro
• cargo test --offline -p manganis
• cargo test --offline -p dioxus-cli opt::tests::
• cargo clippy --offline -p const-serialize -p manganis-core -p manganis-macro --tests --no-deps -- -D warnings
• cargo fmt --package const-serialize --package manganis-core --package manganis-macro --package dioxus-cli -- --check
• git diff HEAD^ --check

Fixes #5777