fix(content-detector): detect and compress space-separated JSON objects

[rohanprichard]

Description

Headroom's detect_content_type() only recognizes content starting with [ as a JSON_ARRAY. Many web search tools (SerpAPI, Tavily, custom backends) return space-separated JSON objects instead of a real array:

{"title": "Result 1", "url": "..."} {"title": "Result 2", "url": "..."} {"title": "Result 3", "url": "..."}

That shape is detected as PLAIN_TEXT (confidence 0.5), so SmartCrusher never processes it and web-search results compress 0% — exactly the high-volume, repetitive output that most benefits from compression.

Closes #1741

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• content_detector.py: _try_detect_json now recognizes a run of ≥2 whitespace-separated (space- or newline-separated) JSON objects and returns JSON_ARRAY with metadata["concatenated"] = True. The router already falls back to the Python regex detector when the native detector returns PLAIN_TEXT (content_router.py), so this fixes routing on the default backend too.
• content_detector.py: added normalize_concatenated_json() (and a _decode_concatenated_json() helper) that rewrites the space-separated shape into a canonical [{…}, {…}] array string.
• smart_crusher.py: SmartCrusher.crush() normalizes concatenated JSON to a real array before handing it to the Rust crusher, so it actually compresses.
• The change is deliberately conservative: a single object stays unclaimed (_try_detect_json('{"id": 1}') → None), and any non-JSON token between objects disqualifies the run. Existing [-array detection is unchanged.
• Added tests and a CHANGELOG entry.

Testing

• Unit tests pass (pytest) — affected suites (full suite has network-dependent ML tests that can't run offline; see note)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom) — not run (mypy not installed in my env; change is fully type-annotated)
• New tests added for new functionality
• Manual testing performed

Test Output

$ ruff check .
All checks passed!

$ pytest tests/test_transforms_content_detection.py -q
............                                                             [100%]
12 passed

$ pytest tests/test_transforms_content_router.py \
         tests/test_smart_crusher_toin_attachment.py \
         tests/test_transforms_tabular.py -q
96 passed, 2 skipped
# + SmartCrusher passthrough tests in test_text_compressors.py: 2 passed

Real Behavior Proof

• Environment: macOS, Python 3.11, headroom-ai==0.29.0 installed from PyPI (ships the prebuilt Rust _core), patched with this change. Default detection backend (native Rust → Python-regex fallback on PLAIN_TEXT).
• Exact command / steps: ran a 100-object space-separated web_search payload through detect_content_type() and ContentRouter().compress(), before and after the patch (repro below).
• Observed result: detection flips PLAIN_TEXT (conf 0.5) → JSON_ARRAY (conf 1.0) and SmartCrusher compression goes from 0.0% to 34.2% (10369 → 6819 bytes) on the identical payload.
• Not tested: the native Rust detector path in isolation (the fix relies on the existing documented Python-regex fallback for PLAIN_TEXT); separators other than whitespace (comma-separated-without-brackets is intentionally not claimed).

Before:

detected  : ContentType.PLAIN_TEXT  conf 0.5
strategy  : CompressionStrategy.SMART_CRUSHER
orig bytes: 10369
comp bytes: 10369
reduction : 0.0%

After:

detected  : ContentType.JSON_ARRAY  conf 1.0
strategy  : CompressionStrategy.SMART_CRUSHER
orig bytes: 10369
comp bytes: 6819
reduction : 34.2%

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (CHANGELOG)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Additional Notes

• Kept as a draft pending a maintainer's read and my own re-run of the repro on a clean install before flipping "ready for human review".
• mypy headroom was not run (mypy isn't in my local env); the new code is fully type-annotated and ruff check . is clean repo-wide.

[github-actions]

PR governance

This draft PR follows the template so far. Keep it in draft until it is ready for human review.