blob_db: fix build failure when FLB_SQLDB is flipped off

[RaJiska]

Building with flag -DFLB_SQLDB=Off would result undeclared types as some gated includes were required for a successful build.

Reproducing:
Add -DDFLB_SQLDB=Off option to the Dockerfile resulting in build error (logs: before_fix.txt).

Changes fix the headers as well as including definitions that were previously missing when SQLDB is turned off, successfully building fluentbit (logs: after_fix.txt)

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Testing
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• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

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• [N/A] Run local packaging test showing all targets (including any new ones) build.
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Backporting

• [N/A] Backport to latest stable release.

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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.

Summary by CodeRabbit

• Bug Fixes
  • Improved compatibility when database-backed features are unavailable.
  • Prevented build and integration issues in configurations without SQL database support.
  • Ensured blob storage operations return consistent results across supported configurations.
• Refactor
  • Aligned optional database-related components with their enabled or disabled configuration.

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes add blob database header dependencies, conditionally include SQLDB support in the systemd configuration, reorder related includes, and complete SQL-disabled blob database stubs with corrected signatures and unavailable-backend return values.

Changes

Blob database compatibility

Layer / File(s)	Summary
Header and conditional include support include/fluent-bit/flb_blob_db.h, plugins/in_systemd/systemd_config.h, src/flb_blob_db.c	Blob database declarations include required types and forward declarations, systemd includes SQLDB support conditionally, and blob database headers precede SQLDB headers.
SQL-disabled blob database stubs src/flb_blob_db.c	The no-backend flb_blob_db_file_part_get_next signature includes part_count, and four missing operations return FLB_BLOB_DB_ERROR_NO_BACKEND_AVAILABLE.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: edsiper

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: fixing build failures when FLB_SQLDB is disabled.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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