feat(boto3): Add SNS extension - #8142

### Description

Propagates the trace through SQS and SNS messages, so the consumer can
be linked back to the request that sent the message.

**Send side** (`SendMessage`, `SendMessageBatch`, `Publish`,
`PublishBatch`):
- Adds `sentry-trace` and `baggage` as `String` message attributes, on
the message or on each batch entry.
- Only when `trace_propagation_targets` match the endpoint.
- Skips a message if the caller already set these attributes, if it
would go over the 10-attribute limit, or if it would go over 256 KiB.
AWS would reject the message otherwise.
- Works on a copy of the params. The caller's dict isn't changed.

**Receive side** (`ReceiveMessage`):
- Adds `sentry-trace` and `baggage` to `MessageAttributeNames`, so SQS
returns them. Only when the span is recorded, and not when the caller
already asks for `All` or `.*`.
- Adds a span link to each distinct producer context in the received
messages, with `sentry.link.type: message_origin`.
- Reads the context from SQS message attributes, and from the JSON
envelope of an SNS notification without raw message delivery.

**Tests**: injection and extraction for single and batch calls, the
limits above, and a round-trip test: send a message, receive what was
injected, and check that the link points at the send span.

Stacked on #8142 (SNS), which is stacked on #8137 (SQS).

#### Issues
* resolves: PY-3008

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)

### 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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… span (#8229)

### Description

The Lambda span set `messaging.batch.message_count` to `1` for every
event that isn't a list, and to `len(event)` for a top-level list. An
SQS or SNS delivery is a dict with `Records`, so a batch of 10 messages
reported 1. HTTP calls and direct invocations reported a batch they
don't have.

This PR removes the attribute from the Lambda span. The count belongs on
messaging spans, as in the OTel messaging conventions:
- boto3 `ReceiveMessage` and batch-send spans already set it (#8137,
#8142).
- For SQS/SNS-triggered Lambdas, the `queue.process` span in #8223 sets
it.

The JS SDK doesn't set it on the Lambda span either. The attribute was
added in 3.0 (#7941) and never shipped in 2.x, so no released behavior
changes.

Replaces #8219 (same commits). GitHub never ran CI on #8219.

**Note for review:** this is narrower than PY-3015, which asks to keep
the attribute for SQS/SNS deliveries. @alexander-alderman-webb, please
confirm that's OK.

#### Issues
* resolves: PY-3015

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