refactor(config): prepare schema-driven value coercion

[pront]

Summary

Extract the schema-coercion groundwork from #25905 so it can be reviewed independently of the breaking parse-first loading change.

• Introduce ValueCoercer to keep the root schema, definition lookup, and error-path state together, with tests in a separate module.
• Cover scalar coercion, references, arrays, unions, generated ConfigBuilder schemas, and numeric boundaries. Reject the rounded 2^63 float boundary instead of silently saturating it to i64::MAX.
• Leave runtime loading unchanged: the loader does not invoke the new coercer. Serde remains the authoritative validator.

References

Related: #25905. Follows merged #26441; based directly on master, not stacked.

How did you test this PR?

• make fmt and cargo fmt --all -- --check passed.
• cargo nextest run -p vector --lib --no-default-features --features sources-demo_logs config::loading: 39 passed, including 12 coercion tests. A repeat run was clean after one passing test was marked leaky on the first run.
• Production-library Clippy passed with sources-demo_logs and warnings denied.
• make check-clippy FEATURES=sources-demo_logs is blocked by the existing clippy::unused_async finding at src/test_util/http.rs:51; that file is unchanged from master.

Does this PR include user facing changes?

• Yes.
• No. A maintainer will apply the no-changelog label to this PR.

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