fix(ado-script): treat unsubstituted ADO macros as empty in synthPr

[jamesadevine]

Summary

Follow-up to #972. The first roll-out of the synthPr real-PR shortcut had a
runtime bug — ADO leaves undefined predefined-variable macros as the
literal string $(name) in step env (it does NOT substitute to empty).
The bundle read

SYSTEM_PULLREQUEST_PULLREQUESTID = "$(System.PullRequest.PullRequestId)"

on every non-PR build, saw a non-empty string, and went down the
real-PR shortcut path. Observed by @jamesadevine in the logs:

[synth-pr] real PR build #$(System.PullRequest.PullRequestId);
 propagating SYSTEM_PULLREQUEST_* to AW_PR_*

The bundle then emitted AW_PR_ID = $(System.PullRequest.PullRequestId) as
a literal, and downstream PR-identifier validation rejected it (same
class of failure as build 612528, just one level deeper).

Fix

Add a resolveAdoMacroEnv helper that treats literal $(name) strings
as empty:

function resolveAdoMacroEnv(value: string | undefined): string {
  if (!value) return "";
  if (/^\$\([^()]+\)$/.test(value)) return "";
  return value;
}

Match is on balanced $( ... ) with no nested parens — ADO macro
names never contain parens, so this is precise. Apply to every
SYSTEM_PULLREQUEST_* env read in exec-context-pr-synth/index.ts so
the bundle correctly falls through to the synth-discovery path on
non-PR builds.

Test plan

• New regression test
  does NOT treat unsubstituted $(System.PullRequest.*) ADO macros as a real PR id
  feeds the exact malformed env from the production log and asserts:
  • "real PR build" log path is NOT taken
  • AW_PR_ID is not contaminated with a literal $(...) string
  • Bundle proceeds to synth-discovery and emits AW_SYNTHETIC_PR_SKIP
    on no match
• npm test — 28/28 synthPr tests pass (1 new, 12 existing).
• npm run build:exec-context-pr-synth — bundle rebuilt successfully.
• No Rust-side changes required: the compiler-emitted YAML is already
  correct (it just passes the macros as env values); the fix is purely
  defensive handling of ADO's unsubstituted-macro behaviour inside the
  consumer bundle.