fix(capture): keep excluded MIME protobuf applicability partial (#6012)

* fix(capture): keep excluded MIME protobuf applicability partial

Automatic protobuf framing treated an explicitly excluded, conflicting or
unavailable Content-Type the same as a known-absent one, so a selected body
was skipped and coverage still upgraded to complete. Body metadata now
reports whether MIME applicability is established; an unresolved non-empty
body adds a record limitation and keeps protobuf_coverage partial. HAR uses
its required mimeType, falling back only to a visible header value.

Fixes #5926

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(capture): compare repeated Content-Type declarations by essence

Repeated Content-Type headers that differ only by case, whitespace or
parameters declare the same media type, yet the raw-string comparison
collapsed them to an unknown value and the new applicability check
reported partial coverage. Body metadata now compares media-type
essences and owns automatic framing selection, so the consumer no longer
re-parses the raw value; framing lookup ignores inherited properties.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/web-network-captures.md

@@ -178,6 +178,9 @@ sanitized source even with explicit sensitivity options. If that source pass
 is interrupted, unchecked bodies are not published as partial Evidence.
 Framing overrides apply only to format-owned body coordinates and preserve
 exclusions for the transport declared in MIME metadata.
+Explicitly excluded or conflicting MIME metadata is not treated as absent:
+automatic framing leaves such a non-empty body uninterpreted, adds a record
+limitation and keeps `protobuf_coverage` partial unless `framing` is selected.
 Unknown or explicitly excluded HTTP coding metadata is not treated as absent:
 SAZ/native bodies remain opaque until identity coding is established. Raw
 header values and retained body bytes remain available in source Evidence.