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

Inline comments:
In `@qdrant_client/connection.py`:
- Around line 176-177: The metadata list is appended with get_context_headers()
causing duplicate keys; update both the sync interceptor block (where metadata
is built from new_metadata/client_call_details.metadata and appended with
get_context_headers()) and the async interceptor block to first remove any
existing entries whose key matches a context header key, then extend the
metadata with context headers; specifically, in the places that reference
metadata, new_metadata, client_call_details.metadata and get_context_headers(),
filter out entries with keys present in get_context_headers().keys() (preserving
order for remaining entries) before adding the context headers so context
headers deterministically overwrite conflicting keys.

In `@qdrant_client/context_headers.py`:
- Around line 5-9: _context_headers currently uses a mutable default dict and
get_context_headers returns the internal dict directly, risking cross-context
mutation; change the ContextVar declaration to use a default of None (or no
default) and update all setters/getters (e.g., get_context_headers,
set_context_headers, and any clear_context_headers) to treat the stored value as
optional, always store copies when calling _context_headers.set(...) and have
get_context_headers return a shallow copy (or an empty dict copy) rather than
the internal object so external mutation cannot taint the ContextVar across
contexts; apply the same copy-on-set/copy-on-get approach to the other related
functions referenced in the diff.
- Around line 34-43: The two middleware functions rest_headers_middleware and
async_rest_headers_middleware lack type annotations and fail mypy; update their
signatures to include explicit types: annotate the request parameter as
starlette.requests.Request (or the project's Request alias), annotate call_next
as Callable[[Request], Response] for the sync middleware and Callable[[Request],
Awaitable[Response]] for the async middleware, and set return types to Response
and Awaitable[Response] respectively; import the needed types (Request,
Response, Callable, Awaitable) and adjust any existing imports accordingly so
both function definitions are fully typed.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Duplicate comments:
In `@qdrant_client/context_headers.py`:
- Around line 7-11: The ContextVar _context_headers currently uses a mutable
default {} and get_context_headers returns the internal dict directly, allowing
external mutation and cross-context leakage; change _context_headers to avoid a
shared mutable default (e.g., no mutable literal default or use None) and update
get_context_headers to return a shallow copy of the stored mapping (or construct
a new dict from the stored value) so callers cannot mutate the internal state;
reference _context_headers and get_context_headers when making the change.