feat(config)!: parse config first, then interpolate into string leaves

[pront]

Summary

Implements RFC #25603: Vector now parses configuration files into a typed value tree before performing environment variable (${VAR}) and SECRET[...] substitution.

Key changes:

• Interpolation operates only on string-typed leaves in the parsed tree, closing a structural injection vulnerability where env var values containing YAML special characters could corrupt the parse
• A JSON-Schema-driven coercion pass converts string scalars to the types declared by each component (int, bool, float), so count = "${MY_COUNT}" with MY_COUNT=42 works correctly
• Removes the old pre-parse string-substitution path (src/config/vars.rs deleted)
• set_env_var_interpolation() / env_var_interpolation_enabled() global OnceLock controls interpolation (same pattern as master's --dangerously-allow-env-var-interpolation flag)
• The HTTP provider, validate, and vector config commands all use the same global flag consistently

Vector configuration

Before (TOML, now rejected):

[sources.in]
type = "demo_logs"
count = ${MY_COUNT}

After (TOML, quoted form works and coerces to int):

[sources.in]
type = "demo_logs"
count = "${MY_COUNT}"

YAML is unaffected: count: ${MY_COUNT} continues to work (YAML parses ${VAR} as a string scalar and the coercion pass converts it to the declared type).

How did you test this PR?

• Unit tests in src/config/loading/interpolation.rs and schema_coercion.rs
• Existing config loading unit tests pass
• Manually validated with cargo run -- validate against YAML/TOML/JSON configs with env var and secret placeholders

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

• Closes: Environment variable substitution and YAML special characters #17343
• Related: feat(config)!: disable env var interpolation by default, add --dangerously-allow-env-var-interpolation #25699 (--dangerously-allow-env-var-interpolation flag)
• RFC: https://github.com/vectordotdev/vector/blob/master/rfcs/2026-06-09-parse-first-config-interpolation.md