Sidecar: isolate downstream API options per request

[soodt]

Sidecar: isolate downstream API options per request

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Summary of the changes (Less than 80 chars)

Description

The sidecar resolves DownstreamApiOptions from IOptionsMonitor.Get(apiName), which is a
process-wide singleton. Both the AuthorizationHeader* and DownstreamApi* endpoints could
apply request-scoped values onto that shared instance:

• the options-override merge (MergeOptions) performs a shallow Clone() and then writes into
  the shared ExtraParameters / ExtraHeaderParameters / ExtraQueryParameters dictionaries, and
• the agent-identity parameters (AgentIdentity / AgentUsername / AgentUserId) are applied
  in place.

Because Clone() is a shallow member copy, those dictionaries are shared with the singleton, so
request-scoped values could persist onto the shared options and affect subsequent requests, or
race under concurrency.

This change isolates the options up front. Both endpoints now call a new
DownstreamApiOptionsMerger.CloneForRequest immediately after resolving the options — before any
merge or agent-identity mutation. CloneForRequest clones the options and allocates fresh
ExtraParameters, ExtraHeaderParameters, and ExtraQueryParameters dictionaries when present,
so every request operates on a fully private copy and the singleton is never mutated.
Override-gating and logging behavior are unchanged, and MergeOptions is untouched. Scopes and
ManagedIdentity are only reassigned (never mutated in place) and are documented as such.

Testing

Adds SidecarOptionsIsolationTests covering: per-request isolation on both endpoints, the merge
path with pre-existing non-null singleton collections, concurrency with distinct agent identities,
and shallow-clone-vs-fresh-dictionary characterization. Full Sidecar.Tests suite passes (the
pre-existing cert/live-AAD integration tests remain environment-gated).