[Fix] Datalayer error handling

[elevran]

What type of PR is this?
/kind bug
/kind documentation

What this PR does / why we need it:
Fix datalayer poll error flooding and improve operator visibility.

• loraspec: fix nil-contract bug - empty loraSpec now correctly disables extraction
• collector: log poll errors only on first occurrence and on recovery (was: silent discard or per-tick flood)
• config: warn at startup when data.sources is an empty list
• test: end-to-end extraction tests via httptest server, covering default config, disabled specs, missing families
• docs: expand data layer configuration section with metrics plugin reference and disable instructions

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

Expanded configuration guide to describe use of "data" section

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: elevran
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