ref(sentry-apps): extract webhook header parsing and masking helpers

[cvxluo]

Move logic for sentry app custom headers reading + masking into its own file, so they can be used by src/sentry/sentry_apps/external_requests/utils.py.

[sentry]

Bug: The parse_custom_headers function will crash with a TypeError if webhook_headers is None, which can happen for older SentryApp database records.
_{Severity: HIGH}

Suggested Fix

Add a guard to the parse_custom_headers function to handle cases where webhook_headers is None. A simple way to do this is to default to an empty list, for example: for header in webhook_headers or []:. This restores the safe behavior that existed before the refactor.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/sentry_apps/utils/headers.py#L8-L9

Potential issue: The function `parse_custom_headers` does not handle `None` values for
its `webhook_headers` argument. This can lead to a `TypeError` when the function
attempts to iterate over `None`. The `webhook_headers` field can be `None` for
`SentryApp` records that were created before the database migration that added this
field, as the migration did not backfill existing rows, leaving them as `NULL` in the
database. The original code handled this possibility with `or []`, but this safeguard
was removed during refactoring, introducing a regression.

Also affects:

• src/sentry/sentry_apps/api/serializers/app_platform_event.py:114~114

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[cvxluo]

pretty sure the column defaults to [] rather than null