fix(asterisk): extract incoming DID/CallerID via AGI instead of broken REGEX()

Asterisk's REGEX() dialplan function only tests for a match (returns 1/0); it
cannot extract a substring. The regex DID and CallerID parsers relied on it, so
didNum/fromCid were set to "1" instead of the captured value and inbound calls
mis-routed.

- Add shared extract_did_cid.php AGI: applies PCRE to a source channel variable
  and writes the captured value into a destination variable (didNum / fromCid).
  Regex is passed base64-encoded so dialplan-special characters ({10,11} comma,
  ';', quotes) survive the extensions.conf parser; the header is read inside the
  AGI to avoid argv splitting.
- Harden the AGI against attacker-controlled headers: cap input length (ReDoS),
  whitelist the result to the inbound-extension charset with the /D modifier
  (rejects embedded comma/newline), and fall back to the whole match when an
  optional capture group matches empty.
- Replace the runtime ${CUT(CONTEXT,-,1-2)} Goto target (broke for provider ids
  with >2 dash segments) with the literal {providerId}-incoming, and guard every
  redirect Goto with a FILTER() charset check so a crafted value cannot split the
  Goto arguments.
- SIPConf: in the shared-context path, build the cid-did context with $context_id
  (not a provider uniqid) so it is reachable by Gosub(${CONTEXT}-cid-did,...) and
  its Goto returns to the correct context.
- Add unit tests covering both DID and CallerID branches.

Author: boffart