Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@pkg/telemetry/phonehome/headers_multimap.go`:
- Around line 54-74: GetMatching on Headers returns nil for a nil/empty Headers
when value == NoHeaderOrAnyValue, causing inconsistent behavior versus the
loop's special-case; update Headers.GetMatching so that if h is nil (or empty)
and value == NoHeaderOrAnyValue it returns an empty map instead of nil, e.g. in
the beginning of GetMatching check if value == NoHeaderOrAnyValue and return
make(map[string][]string) when h == nil (and then remove/keep the in-loop
initialization accordingly); reference: Headers.GetMatching, NoHeaderOrAnyValue,
GetMatchingValues.

In `@pkg/telemetry/phonehome/request_params.go`:
- Around line 34-45: RequestParams.MatchHeaders currently uses maps.Copy to
merge per-pattern matches which causes later patterns to overwrite earlier
concrete header entries (non-deterministic for overlapping globs). Replace the
maps.Copy(result, matching) behavior in RequestParams.MatchHeaders with a
per-key merge: iterate over each concrete header key returned by GetMatching and
if that header already exists in result, combine/union the values instead of
replacing them; ensure the merge semantics match the Headers type (e.g., append
or deduplicate values). Alternatively, add a validation step when constructing
the GlobMap to detect and reject overlapping header-name patterns (X-* vs
X-Test) so MatchHeaders can keep deterministic behavior; reference
RequestParams.MatchHeaders, GetMatching, Headers, GlobMap and NoHeaderOrAnyValue
when implementing the fix.