…pan.source` attribute (#6835)

Following the
[deprecation](https://getsentry.github.io/sentry-conventions/attributes/sentry/#sentry-span-source)
of `sentry.span.source`, this PR renames the attribute from
`sentry.span.source` to
[`sentry.segment.name.source`](https://getsentry.github.io/sentry-conventions/attributes/sentry/#sentry-segment-name-source).

---------

Co-authored-by: Erica Pisani <[email protected]>

### Description
Add span streaming variants to tests in `test_http_headers`


#### Issues
Part of #5395

#### 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](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

### Description
Reverting `sampling_context` to look like it used to in
transaction-based tracing.

Caveat: There's a bunch of keys that are not populated in span
streaming, mostly because they're not known at span start (`span_id`,
`sampled`, etc.). I've kept them in as `None` as that's how the legacy
path did it, too (rather than omitting them).

Note: When bumping the sdk version in sentry and seer to a version that
has this change, the sampling_context code needs to change there, too.

#### Issues
resolves #6846

#### 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](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

…sts (#6855)

## Summary
- Replaced all `_experiments={"trace_lifecycle": ...}` with top-level
`trace_lifecycle=...` across 77 test files
- Replaced `_experiments={"ignore_spans": ...}` with top-level
`ignore_spans=...`
- Kept backwards-compat tests in `test_client.py` and
`test_span_streaming.py` that specifically verify `_experiments` still
works and that top-level takes precedence

Co-authored-by: Claude Opus 4.6 <[email protected]>

…r callbacks (#6853)

When a user-provided traces_sampler callback raises, catch the
exception,
log a warning, and fall back to the parent sample rate or the configured
traces_sample_rate instead of propagating the error.

Wrap the before_send callback invocation in
capture_internal_exceptions()
so that exceptions raised by user-provided before_send_log,
before_send_metric, and before_send_span callbacks do not crash the
application. When a callback raises, the original, unmodified telemetry
item is sent.

Fixes PY-2617
Fixes #6850

…streaming trace lifecycle (#6845)

Add to `Scope.start_transaction()` the same deprecation warning emitted by `Scope.start_span()`, and no-op when the function is used together with the streaming trace lifecycle.

Update tests that incorrectly assert on transactions while that lifecycle is enabled.

… lifecycle (#6844)

Do not set transaction-based spans on the scope when the streaming trace lifecycle is enabled. Early returns in `start_span()` and `start_transaction()` return a `NoOpSpan`, whose `__enter__()` and `__exit__()` methods still mutate the scope.