Implement multi-upstream sequential authorization chain

[jhrozek]

Summary

• The auth server previously supported only a single upstream IDP. This PR implements RFC-0052 Phase 2: a sequential authorization chain that transparently redirects users through every configured upstream IDP before issuing a single authorization code. This enables scenarios like corporate-idp + github + atlassian requiring tokens from all three providers.
• Refactors Handler from a single upstream field to an ordered []NamedUpstream slice, wires SessionID generation into AuthorizeHandler (threaded across all chain legs), and adds continueChainOrComplete logic in CallbackHandler that either redirects to the next upstream or issues the final auth code.
• Includes defense-in-depth: identity consistency verification across chain legs, cleanup of upstream tokens on any chain failure, fresh PKCE/state/nonce per leg, and IDP mix-up protection via upstreamByName lookup.

Closes #4137

Type of change

• New feature

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• Manual testing: this PR is cherry-picked from a larger integration branch (thv-as-vmcp) where the full multi-upstream flow was tested end-to-end with real OIDC providers against the Entra OBO test environment.

Changes

File	Change
pkg/authserver/server/handlers/handler.go	Replace single upstream field with []NamedUpstream slice; add NamedUpstream type, nextMissingUpstream, upstreamByName helpers; harden NewHandler constructor with validation
pkg/authserver/server/handlers/authorize.go	Generate SessionID at chain start; target upstreams[0] as first leg; use slice-based provider lookup
pkg/authserver/server/handlers/callback.go	Add continueChainOrComplete chain logic; carry identity from first leg through ResolvedUser* fields; add identity mismatch detection; clean up tokens on all failure paths
pkg/authserver/server/handlers/user.go	Downgrade user creation log from Info to Debug
pkg/authserver/server_impl.go	Build ordered upstream slice from all cfg.Upstreams; pass to NewHandler; update UpstreamTokenRefresher to dispatch by provider name
pkg/authserver/config.go	Remove len > 1 guard on upstreams; update doc comments
pkg/authserver/refresher.go	Accept provider map for multi-upstream token refresh dispatch
pkg/authserver/server/handlers/handler_chain_test.go	New: tests for nextMissingUpstream (all-satisfied, first-missing, partial, storage error)
pkg/authserver/server/handlers/callback_test.go	New: 8 multi-upstream chain tests (first-leg redirect, second-leg code issuance, identity carry-forward, identity mismatch rejection, fresh secrets, auth URL error cleanup, store-pending error cleanup)
pkg/authserver/integration_test.go	New: full sequential chain integration test with two mock OIDC providers (authorize → leg-1 callback → leg-2 callback → token exchange)
pkg/authserver/server/handlers/helpers_test.go	Add multiUpstreamTestSetup, baseTestSetupOption, multi-provider storage mock wiring

Does this introduce a user-facing change?

Multiple upstream IDPs can now be configured. The auth server will sequentially authorize through each before issuing a token.

Special notes for reviewers

• Identity resolution strategy: The first upstream in the chain is the "identity provider" — it resolves/creates the internal user. Subsequent legs carry the resolved identity forward via PendingAuthorization.ResolvedUser* fields rather than re-resolving.
• nextMissingUpstream returns error (not fail-safe restart): The issue spec suggested returning upstreamOrder[0] on storage error. The implementation instead returns the error, allowing the caller to clean up and return a proper error to the client. A silent restart could loop indefinitely if storage is persistently down.
• 302 vs 303 as chain signal: Chain-continuation redirects use 302 (Found) to the next upstream, while the final auth code redirect uses fosite's 303 (See Other). Tests assert on this distinction.

Large PR Justification

The size owes itself mostly to tests

Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 77.38095% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.18%. Comparing base (f52b6d0) to head (ea954ab).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/authserver/server/handlers/callback.go	74.22%	20 Missing and 5 partials ⚠️
pkg/authserver/server_impl.go	74.07%	5 Missing and 2 partials ⚠️
pkg/authserver/server/handlers/handler.go	81.81%	2 Missing and 2 partials ⚠️
pkg/authserver/server/handlers/authorize.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4283      +/-   ##
==========================================
+ Coverage   68.54%   69.18%   +0.64%     
==========================================
  Files         471      471              
  Lines       47732    47761      +29     
==========================================
+ Hits        32717    33045     +328     
+ Misses      12247    12149      -98     
+ Partials     2768     2567     -201     

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I explained the size, go away now

[jhrozek]

Thanks for the review, I'll add the fixups to the next PR in the interest of moving fast.