Post-review follow-ups for Graph send (#825/#826) and requests-to-httpx migration (#827)

Follow-ups from the review of PR #825 (whose implementation had already
landed on master via #826's stacked merge):

- Honor the documented [smtp] attachment and [smtp] message options.
  Both were parsed into opts but never passed to either summary-email
  transport (also broken in released 10.2.2), so a configured custom
  attachment filename or message body was silently ignored. Both the
  SMTP and Microsoft Graph transports now receive them, and the missing
  smtp_attachment Namespace default is added (also covers SIGHUP
  reload, which rebuilds opts from the CLI Namespace).
- Don't mislabel non-Graph mailbox errors as Microsoft Graph failures:
  the shared mailbox-fetch and watch handlers now log a generic
  "Mailbox Error" with traceback when the connection isn't Graph.
- Declare microsoft-kiota-abstractions as a direct dependency (imported
  directly in cli.py for Graph error handling; previously transitive).

Migrate all runtime HTTP from requests to httpx (webhook client, Splunk
HEC client, and the PSL-overrides / IP-database / reverse-DNS-map /
IPinfo-API fetches in utils.py):

- follow_redirects=True everywhere to preserve requests' default
  redirect-following; httpx does not follow redirects by default.
- The PSL-overrides and reverse-DNS-map fetches gain a 60s timeout
  (previously none), matching the IP-database fetch.
- response.ok -> response.is_success; requests.RequestException ->
  httpx.HTTPError; raw string bodies use content= (httpx's data= is
  form-encoding only); Splunk HEC verification moves to client
  construction (httpx has no per-request verify).
- requests drops out of [project] dependencies and moves to the [build]
  extra for the out-of-wheel maintainer script collect_domain_info.py,
  which deliberately stays on requests/urllib3 for its permissive-TLS
  adapter.
- Remove the requests-era module-level
  urllib3.disable_warnings(InsecureRequestWarning) in splunk.py; httpx
  doesn't route through urllib3, so its only remaining effect was
  globally silencing insecure-TLS warnings from other urllib3-based
  components as an import side effect. Nothing imports urllib3 directly
  anymore, so it also leaves [project] dependencies.

Tests: config-to-transport wiring for attachment/message on both
transports (including defaults), non-Graph errors keep the generic log
line, webhook/Splunk payload assertions moved to content=, and Splunk
verify asserted at httpx.Client construction. 736 passed; ruff and
pyright clean.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/source/usage.md

@@ -546,8 +546,9 @@ The full set of configuration options are:
   - `subject` - str: The Subject header to use in the email
     (Default: `parsedmarc report`)
   - `attachment` - str: The ZIP attachment filenames
+    (Default: `DMARC-<YYYY-MM-DD>.zip`)
   - `message` - str: The email message
-    (Default: `Please see the attached parsedmarc report.`)
+    (Default: `Please see the attached DMARC results.`)
 
     :::{note}
     `%` characters must be escaped with another `%` character,