conditionals: resolve TAG record accessor in processor and filter conditions

[zshuang0316]

Summary

Condition rules used by processors and filters could not match on the record
tag. The record accessor already parses $TAG / $TAG[n], but the condition
evaluator called flb_cfl_ra_translate() with a NULL tag, so any rule
referencing $TAG silently never resolved.

This PR threads the chunk tag through the condition-evaluation path:

• flb_condition_evaluate() / flb_condition_evaluate_ex() and evaluate_rule()
  gain tag/tag_len arguments and forward them to flb_cfl_ra_translate().
• struct flb_mp_chunk_cobj carries the chunk tag/tag_len; flb_processor
  sets them around the processor callback and clears them afterwards.
• The record-next path (flb_mp) and the filter path pass the chunk tag; the
  router passes NULL since it already routes by tag natively.

This enables conditions such as:

    processors:
      logs:
        - name: content_modifier
          action: insert
          key: matched_tag
          value: "true"
          condition:
            op: and
            rules:
              - field: "$TAG"
                op: regex
                value: "^app\.frontend\..*$"

so a processor or filter can select records by tag.

The change is split into per-subsystem commits (conditionals:, mp:,
processor:, router_condition:, tests:) to satisfy the commit-prefix
convention; the tree builds as a whole.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change (see above)
• Debug log output from testing the change
• Attached Valgrind output that shows no leaks or memory corruption was found

Unit coverage is added in tests/internal/conditionals.c (tag_accessor):
exact-match, regex, non-matching tag, and the no-tag case. I'll follow up with
runtime debug/Valgrind logs.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[coderabbitai]

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[zshuang0316]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 61420063b3

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[zshuang0316]

Converting this to a draft. Opened #12103 to confirm the design direction first — whether $TAG should be resolvable inside processor/filter conditions, and if so whether threading tag/tag_len through flb_condition_evaluate* is the shape maintainers prefer. This PR stands as a reference implementation for that discussion; I'll iterate or close based on the outcome.