chore: add changeset for per-request overrides feature

Author: kwardega-sc

.changeset/add_per_request_headers_timeout_auth_overrides.md

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+---
+default: minor
+---
+
+# Add per-request headers, timeout, and auth overrides to endpoint functions
+
+All generated endpoint functions (`sync_detailed`, `asyncio_detailed`, `sync`, `asyncio`) now accept three optional keyword arguments forwarded directly to the underlying httpx request:
+
+- `headers: dict[str, str] | None = None` — extra headers merged on top of any spec-defined headers for this request
+- `timeout: httpx.Timeout | None | Unset = UNSET` — override the client-level timeout for this request; `None` disables the timeout
+- `auth: httpx.Auth | None | Unset = UNSET` — override the client-level auth for this request; `None` disables auth
+
+This allows per-request customisation without creating a new client instance, preserving the shared httpx connection pool. Using `with_headers()` / `with_timeout()` at runtime was previously the only option, but those methods mutate the original client's underlying httpx client as a side effect and cause the returned client to open a new connection pool on first use—making them unsafe for concurrent async code.
+
+Note: client-level headers cannot be fully removed for a single request via this mechanism, only overridden with a different value.