chore(confinement): create UnconfinedTemplate and Confined<> abstractions

[thomasqueirozb]

Summary

Introduces UnconfinedTemplate and Confined<T> as first-class types to make the path-confinement contract explicit at the type level across sinks. Sink config structs now store UnconfinedTemplate; sinks call .confine() in build() to obtain a Template (Confined<UnconfinedTemplate>) that enforces the confinement invariant at render time.

Also fixes a series of RUST_STYLE.md violations and correctness bugs identified during review:

• use statements inside function bodies moved to top-level imports
• warn! calls converted to tracing key/value style
• Inline format string arguments converted to {var} syntax
• assert! → debug_assert! in Confined::confine
• pub(crate) inner on Confined<T> narrowed to private
• Confined<UnconfinedTemplate>::metadata() delegates to UnconfinedTemplate::metadata() to preserve docs::templateable in generated schema
• compute_templated_field_keys extracted as a shared free function in splunk_hec/metrics
• Hardcoded component name string replaced with Self::NAME in redis/config.rs

Vector configuration

NA

How did you test this PR?

make check-clippy passes.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

NA