Skip analytics for configured ignore header

[Gutts-n]

What changed

AnalyticsMiddleware now skips analytics entirely for a request whose
configured header matches a configured value - not logged with a
distinguishing field, just never recorded. New config:

• ANALYTICS_IGNORE_HEADER - header name to check (e.g. Request-Source)
• ANALYTICS_IGNORE_VALUES - comma-separated values that trigger the skip
  (e.g. data-explorer), matched case-insensitively

Both empty (the default) disables the check - every tracked request is
logged, same as before this PR. The check runs before any capture/body-tee
work, so a matched request costs nothing extra.

Why

Part of the NESO API-usage-metrics work (neso-nextgen-shared#152). Data
Explorer sends Request-Source: data-explorer on its calls to this service
(neso-nextgen-shared#145). Original version of this PR captured the header
into the event and left filtering to the BigQuery/dashboard layer - @sagargg
pointed out
that's the wrong layer: better to not log this traffic at all than log it
and filter downstream. Reworked accordingly.

Same change going into ckanext-analytics (CKAN Action API side). The
neso-monitoring dashboard PR's BigQuery filter is now unnecessary and will
be reverted separately.

Verified

• pytest - 484 passed.
• ruff check / mypy - clean.
• Tests cover: header+value match skips (case-insensitive), no header sent
  still records, header present with a non-matching value still records,
  check disabled by default (unconfigured) never skips.

Not verified: end-to-end against a live pipeline.

[coderabbitai]

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