fix(adcp): skip eager httpx.AsyncClient alloc in WebhookSender.__aenter__ on owned-client path

[bokelley]

Closes #300

When WebhookSender owns its client (_owns_client=True, i.e. no caller-supplied client=), __aenter__ was unconditionally calling _get_client(), which allocated an httpx.AsyncClient that sat idle until __aexit__. On the owned-client path, _send_bytes calls _get_client() lazily at first use anyway, so the eager allocation in __aenter__ was wasted. The fix skips it; the operator-supplied-client branch (_owns_client=False) still calls _get_client() for symmetry, though that call is a no-op since self._client is already set.

What was tested

• pytest tests/ -k "webhook_sender or webhook" — 182 passed, 0 failed
• ruff check src/ — clean
• mypy src/adcp/webhook_sender.py — no new errors (pre-existing errors are in unrelated files)

Pre-PR review

• code-reviewer: approved — fix correct and non-breaking; nit: _get_client() call on _owns_client=False branch is a harmless no-op but could be removed for clarity.
• dx-expert: approved — both async with WebhookSender(...) paths (owned and operator-supplied) preserve their existing contract; no docstring update needed.

Triage-managed PR. This bot does not currently iterate on
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issue). To unblock:

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  issue.

See adcp#3121
for context.

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