opentelemetry-instrumentation-httpx: support strict typing

[henry3260]

Description

Add opentelemetry-instrumentation-httpx to the pyright strict typecheck
environment (tox.ini typecheck deps + pyright include in pyproject.toml)
and fix the typing across the instrumentation. Follow-up to the review
discussion in #4730.

Notable changes, none of which are intended to change runtime behavior:

• Restructure the isinstance dispatch in _extract_response (check tuple
  first) so pyright narrowing works soundly in both branches; this removes the
  previously flagged unnecessary typing.cast calls.
• Declare the transport handle_request/handle_async_request methods as
  returning httpx.Response to be compatible with the httpx.BaseTransport
  override; the httpx < 0.20 tuple form is still handled at runtime.
• Dispatch sync/async hooks in instrument_client via TypeIs guards
  (pyright does not narrow callable unions in the negative direction, so a
  pair of positive guards is used).
• Type span/metric attribute dicts as dict[str, AttributeValue] and make the
  wrapper histogram/hook parameters optional to match call sites.
• Widen RequestInfo/ResponseInfo url and extensions field annotations
  to match the raw values the transport layer actually passes.

Remaining suppressions: two typing.cast calls at the transport returns
(control-flow invariant pyright cannot see, documented inline) and two
targeted pyright: ignore comments for untyped wrapt and the deprecated
MetricInstruments name required by the old semconv mode — both follow
existing precedent in already-typechecked packages.

Package tests remain excluded from typecheck for now, consistent with the
other packages in the include list.

Fixes #4818

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• tox -e typecheck — 0 errors for the package
• tox -e lint-instrumentation-httpx — 10.00/10
• tox -e py312-test-instrumentation-httpx-{0,1,2,3}-wrapt2,
  py312-test-instrumentation-httpx-1-wrapt1, and
  py310-test-instrumentation-httpx-1-wrapt2 — all pass; the -0 factor
  (httpx 0.18) exercises the legacy tuple code path touched by the
  _extract_response restructuring

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added (typing-only change; covered by the existing test suite)
• Documentation has been updated (not applicable)