enhancement(remap): unify VRL source configuration

[pront]

Summary

The remap transform had three mutually exclusive source fields (source, file, files)
with no schema-level enforcement: supplying more than one was caught only at runtime with an
opaque error, and the docs generator had to strip docs::examples from the alternatives to
avoid emitting invalid auto-generated configs. This consolidates everything under a single
source field whose object form is a typed, AnyCondition-style enum, so the constraint is
enforced at parse time and generated docs carry accurate examples again. source stays the
primary field because nearly every remap config uses it.

Changes

• source now accepts an inline VRL string (unchanged, the common case) or a type-tagged
  object: { type: vrl, program: "..." } for an inline program, or { type: disk, files: [...] }
  to load from disk.
• A custom deserializer enforces that exactly one of source, file, or files is set (also on
  the compile path, for configs built directly in Rust) and preserves each field through
  serialization so configs round-trip.
• The top-level file and files fields are deprecated: still functional, documented as
  deprecated, and they emit a startup warning pointing to the new form.

Vector configuration

Inline (unchanged):

source: |
  .foo = "bar"

Inline, explicit form:

source:
  type: vrl
  program: |
    .foo = "bar"

Load from disk:

source:
  type: disk
  files:
    - path/to/program_1.vrl

Deprecated, still works:

file: path/to/program_1.vrl

How did you test this PR?

• Unit tests covering deserialization of every form, mutual-exclusivity errors, empty-source
  round-tripping, and end-to-end compile/execute for both type: vrl and type: disk (the disk
  case also asserts file watching).
• cargo run -- validate on configs for all forms, including deprecated file/files (validate
  and emit the warning) and a source + file conflict (fails at load with must provide exactly one of source, file, or files).
• make validate-config-examples for the generated website examples.

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: Investigate a proper way to declare mutually exclusive config fields #25945

[datadog-vectordotdev]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 67d0872 | Docs | Datadog PR Page | Give us feedback!}

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 021ec10e8f

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 9287a95d99

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 872ba5dc18

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".