feat(aws-lambda): Detect Lambda triggers - #8222

### Description

With #8117 (merged), a synchronous boto3 `Invoke` carries `sentry-trace`
and `baggage` in `ClientContext.custom`. This PR adds the receiving
side: the Lambda integration continues the trace from
`context.client_context.custom`. `ClientContext` wins over event
headers, because a forwarded HTTP event can still carry headers from
further upstream.

`sam local` drops `ClientContext`, so the new `ClientContextTrace` test
function hands the event's `client_context` to the runtime. That way the
container tests can cover it.

This is Pablo's code from `pablo/aws-lambda-client-context-trace`. The
one change: `client_context` and `custom` are read with `getattr`, so a
context object without them falls back to event headers instead of
failing. It's the first of three stacked PRs for the stretch Lambda
work: this PR → trigger detection (#8222) → `queue.process` spans
(#8223).

#### Issues
* resolves: PY-3009 (receiving side; #8117 does the injection)
* Part of #8071

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via @dingsdax.
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Co-authored-by: Pablo Deputter <pablo.deputter@sentry.io>
Co-authored-by: Pablo Deputter <71842639+pabloDeputter@users.noreply.github.com>
Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Johannes Daxböck <114897+dingsdax@users.noreply.github.com>

)

### Description

Since #8137, #8142, and #8145, boto3 puts `sentry-trace` in SQS and SNS
message attributes, but nothing reads it on the Lambda side. This PR is
the smallest change that closes the loop for SQS- and SNS-triggered
functions. The Lambda integration now reads `sentry-trace` from each
record and adds a `message_origin` link from the invocation span to each
distinct producer span. This matches what the boto3 `ReceiveMessage`
span already does. Messages never become the invocation's parent.

It reads:
- SQS records: `messageAttributes["sentry-trace"].stringValue`
- SNS records: `Sns.MessageAttributes["sentry-trace"].Value`
- SNS → SQS without raw message delivery: the SNS envelope in the SQS
`body`, the same as boto3 does

Links are deduplicated, because a batch send gives many messages the
same producer span, and capped at 128, OTel's default, because Lambda
can deliver up to 10,000 SQS messages at once.

**Scope.** This is intentionally kept separate from the stretch stack.
It doesn't add `faas.trigger` (#8222) or `queue.process` spans (#8223),
and it doesn't touch `messaging.batch.message_count` (#8229). The
invocation span is otherwise unchanged. Synchronous `Invoke` propagation
through `ClientContext` is #8221. That PR changes the `continue_trace`
call and this one only adds `links=`, so the two are independent.

The link-building code repeats boto3's `_message_link` in a Lambda-event
shape: Lambda records use `messageAttributes` / `stringValue` / `body`,
while the boto3 response uses `MessageAttributes` / `StringValue` /
`Body`. Importing from the boto3 integration would make `aws_lambda`
depend on botocore.

**Testing.** `test_producer_links` sends SQS, SNS, and SNS-to-SQS events
to `BasicOkSpanStreaming`. It asserts the segment span's `links` and
that the segment span has no parent. It passes in CI's Cloud (Lambda)
jobs, and `ruff` and `mypy sentry_sdk` pass. `codecov/patch` is red
because the Lambda tests run in `sam local` containers, so their
coverage isn't counted.

#### Issues
* Part of #8081

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via **webb**, @sentrivana.
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---------

Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Alexander Webb <alexander.webb@sentry.io>
Co-authored-by: Ivana Kellyer <131587164+sentrivana@users.noreply.github.com>