fix(exec): honor Env override and keep secrets out of debug logs

[zxyao145]

• Clear ProcessStartInfo.Environment before applying CodexOptions.Env, so the override replaces the parent environment as documented instead of extending it
• Log environment variable names only when starting the CLI; values (including CODEX_API_KEY) are no longer written to the debug log
• Move CodexExec logging to [LoggerMessage]; the start message is only formatted when Debug is enabled
• Resolve the executable path once per CodexExec instead of on every run (on Windows it probes every PATH directory), and use Environment.ProcessPath instead of Process.GetCurrentProcess().MainModule
• Add ApplyEnvironment tests for the override and inherit cases; FakeCodexCli now passes the parent environment explicitly, since the fake CLI is a .NET apphost that needs it to locate the runtime

Behavior change

Callers that set CodexOptions.Env no longer inherit the parent environment (PATH, HOME, DOTNET_ROOT, ...). Anyone relying on the old behavior must now include those variables in Env. The debug log placeholder {Environment} is renamed to {EnvironmentKeys}.

Before the fix, the new test reproduces the leak:

failed CodexSdk.Tests.CodexExecTests.ApplyEnvironment_WhenEnvOverrideProvided_DoesNotInheritProcessEnvironment
  Assert.Equal() Failure: Collections differ at index 0
  Expected: "CODEX_API_KEY"
  Actual:   "AI_AGENT"

Validation

dotnet test --solution codexsdk.slnx: total 52, failed 0, succeeded 52, skipped 0.

Not included

System.Text.Json source generation for event parsing was evaluated and dropped: because the event DTOs use init accessors, it was ~15–20% slower than reflection and allocated more per event.