ref(httpx,httpx2): Move crumbs to integrations

[sentrivana]

Description

Move breadcrumbs from the centralized, span-powered maybe_create_breadcrumbs_from_span directly to the HTTPX and HTTPX2 integrations. (Put the two together in one PR since they're the same changeset.)

Additional changes and context:

• Running the async breadcrumb tests with pytest-asyncio, as that simulates how the scopes behave live better than setting up an ad-hoc event loop.
• Split the crumb tests into span streaming/not span streaming (because of the difference in send_default_pii behavior).

Issues

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[sentrivana]

The is_not_span_streaming_enabled part is there for continuity in transaction mode, where we don't care about should_send_default_pii before setting breadcrumb data.

[github-actions]

Codecov Results 📊

✅ 112111 passed | ⏭️ 6749 skipped | Total: 118860 | Pass Rate: 94.32% | Execution Time: 401m 48s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +648
Passed Tests	📈 +648
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2487 uncovered lines.
✅ Project coverage is 90.19%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    90.17%    90.19%    +0.02%
==========================================
  Files          193       193         —
  Lines        25337     25353       +16
  Branches      9286      9294        +8
==========================================
+ Hits         22846     22866       +20
- Misses        2491      2487        -4
- Partials      1429      1428        -1

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Generated by Codecov Action

[sentry-warden]

HTTPX breadcrumbs are silently skipped when the request raises

When real_send raises an exception, the new add_http_breadcrumb call after the span block is unreachable, so no HTTP breadcrumb is recorded for failed requests. Previously Span.__exit__ created this crumb during cleanup regardless of exceptions.

Evidence

• Span.__exit__ in tracing.py unconditionally calls self.finish(), which in turn calls maybe_create_breadcrumbs_from_span, so the old span-based mechanism always emitted a crumb—even when the request raised.
• The new code places breadcrumb_data = { ... } and add_http_breadcrumb(...) after the if/else span blocks, so any exception propagated by real_send bypasses it entirely.
• This means network errors, timeouts, or any other HTTP library exception inside the span block silently drop the breadcrumb, losing telemetry context that was previously available.

_{Identified by Warden · code-review · B7U-CFR}