Credential prompt abort shouldn't modify the cache

Requires:
- Approval/rejection (#23711)
- Allow prompt keyword (#23690)

Helpful:
- Generate tests (#23668)

I would apply this commit to the approval/rejection PR

test/libgit2.jl

@@ -2053,12 +2053,12 @@ mktempdir() do dir
             invalid_password = randstring(15)
             invalid_cred = LibGit2.UserPasswordCredentials(invalid_username, invalid_password)
 
-            function gen_ex(; cached_cred=nothing)
+            function gen_ex(; cached_cred=nothing, allow_prompt=true)
                 quote
                     include($LIBGIT2_HELPER_PATH)
                     cache = CachedCredentials()
                     $(cached_cred !== nothing && :(LibGit2.approve(cache, $cached_cred, $url)))
-                    payload = CredentialPayload(cache)
+                    payload = CredentialPayload(cache, allow_prompt=$allow_prompt)
                     err, auth_attempts = credential_loop($valid_cred, $url, "", payload)
                     (err, auth_attempts, cache)
                 end
@@ -2075,7 +2075,6 @@ mktempdir() do dir
                 "Username for 'https://github.com':" => "$valid_username\n",
                 "Password for 'https://$valid_username@github.com':" => "$valid_password\n",
             ]
-
             err, auth_attempts, cache = challenge_prompt(ex, challenges)
             @test err == git_ok
             @test auth_attempts == 1
@@ -2093,6 +2092,27 @@ mktempdir() do dir
             @test auth_attempts == 2
             @test typeof(cache) == LibGit2.CachedCredentials
             @test cache.cred == Dict(cred_id => valid_cred)
+
+            # Canceling a credential request should leave the cache unmodified
+            ex = gen_ex(cached_cred=invalid_cred)
+            challenges = [
+                "Username for 'https://github.com' [alice]:" => "foo\n",
+                "Password for 'https://[email protected]':" => "bar\n",
+                "Username for 'https://github.com' [foo]:" => "\x04",
+            ]
+            err, auth_attempts, cache = challenge_prompt(ex, challenges)
+            @test err == abort_prompt
+            @test auth_attempts == 3
+            @test typeof(cache) == LibGit2.CachedCredentials
+            @test cache.cred == Dict(cred_id => invalid_cred)
+
+            # An EAUTH error should remove credentials from the cache
+            ex = gen_ex(cached_cred=invalid_cred, allow_prompt=false)
+            err, auth_attempts, cache = challenge_prompt(ex, [])
+            @test err == eauth_error
+            @test auth_attempts == 2
+            @test typeof(cache) == LibGit2.CachedCredentials
+            @test cache.cred == Dict()
         end
 
         @testset "Incompatible explicit credentials" begin