refactor: consume datazoo-oauth2 instead of carrying our own copy

[jrosskopf]

Pays off the deferred half of the extraction: the OAuth2 stack that was lifted out of this repo into DataZooDE/datazoo-oauth2 is now consumed back from it, pinned by commit. There is one implementation, not two.

What changed

• 16 files deleted here — oauth2_types, oauth2_flow_v2, oauth2_server, oauth2_browser, oauth2_callback_handler, http_client, timeout_http_client, charset_converter and their headers
• 61 includes repointed at datazoo/oauth2/
• datazoo-oauth2 added as a submodule, pinned at 91c18f9 (v0.1.1-2)

Net: 74 files changed, 97 insertions, 3430 deletions.

The gate: existing OAuth2 suites pass UNCHANGED

If a test had needed editing, the extraction would not have been behaviour-preserving. None did — the only edits anywhere under test/ are include paths, no assertion is touched, and test_datasphere_oauth2_consolidated.cpp / test_microsoft_entra_auth.cpp are otherwise byte-identical.

C++ suite   1141 assertions, 226 cases, all passing
live        graph_entra, graph_planner, graph_sharepoint, graph_excel,
            graph_outlook, microsoft_entra, business_central — green against real tenants
skipped     graph_teams_integration, business_central_integration (environment, not code)

Rebased onto current main

The branch had been cut before #53 landed and carried a duplicate copy of its own telemetry commit (feat(telemetry): adopt schema-2 telemetry). Rebasing onto origin/main dropped that duplicate automatically, leaving the three commits above.

Re-verified after the rebase: builds clean, and the C++ suite still reports exactly 1141 assertions in 226 cases — the same numbers recorded when the migration was first written, which is what makes it safe to say the rebase changed nothing.

The duckdb submodule pin is untouched.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}