Optimizations for nidx extracted text API

[jotare]

Description

Describe the proposed changes made in this PR.

How was this PR tested?

Describe how you tested this PR.

[codecov]

Codecov Report

❌ Patch coverage is 59.80861% with 84 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.06%. Comparing base (dc87887) to head (3c9931b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
nidx/nidx_text/src/reader.rs	64.76%	68 Missing ⚠️
nidx/src/searcher/shard_text.rs	0.00%	15 Missing ⚠️
nidx/nidx_text/src/lib.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3597      +/-   ##
==========================================
- Coverage   85.10%   85.06%   -0.04%     
==========================================
  Files         552      552              
  Lines       47151    47321     +170     
  Branches    13467    13637     +170     
==========================================
+ Hits        40126    40255     +129     
- Misses       6434     6477      +43     
+ Partials      591      589       -2     

Flag	Coverage Δ
nidx	79.73% <59.80%> (-0.05%)	⬇️
nucliadb	73.07% <ø> (ø)
nucliadb-ingest	43.59% <ø> (-0.02%)	⬇️
nucliadb-reader	43.70% <ø> (+0.01%)	⬆️
nucliadb-search	54.09% <ø> (ø)
nucliadb-standalone	46.06% <ø> (ø)
nucliadb-train	44.72% <ø> (ø)
nucliadb-writer	46.99% <ø> (-0.01%)	⬇️
nucliadb_dataset	73.76% <ø> (ø)
nucliadb_models	71.63% <ø> (ø)
nucliadb_sdk	83.49% <ø> (ø)
nucliadb_telemetry	71.45% <ø> (ø)
nucliadb_utils	80.61% <ø> (ø)

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