[client] Name the resolving reader for what it does, not what it makes

ReadOrGenerateConfig reads the profile config and falls back to the defaults in
memory when there is no file. "Generate" reads as "produces and stores", which
is the opposite of the property the rename it came from was meant to advertise:
the read is pure, writes nothing and mints no identity.

ReadConfigOrDefault says the same without the side effect, and pairs with
GetExistingConfig, which fails where this one falls back. Its doc comment now
states the absence of a write rather than only the fallback.

Pure rename; the two remaining mentions of the pre-branch name ReadConfig in
the tests go with it.

Reported by pappz in review.

client/internal/profilemanager/config.go

@@ -1330,9 +1330,11 @@ func CreateInMemoryConfig(input ConfigInput) (*Config, error) {
 	return createProvisionedConfig(input)
 }
 
-// ReadOrGenerateConfig reads the profile config at configPath if it exists, or
-// generates one in memory from the defaults.
-func ReadOrGenerateConfig(configPath string) (*Config, error) {
+// ReadConfigOrDefault reads the profile config at configPath, or resolves the
+// default config in memory when the file does not exist. It never writes, and
+// never mints an identity — EnsureIdentity is where that happens, so the
+// caller that provisions is also the one that persists.
+func ReadConfigOrDefault(configPath string) (*Config, error) {
 	return readConfig(configPath, true)
 }
 
@@ -1457,7 +1459,7 @@ func ConfigToJSON(config *Config) (string, error) {
 //
 // The peer identity is deliberately none of its business, in either direction.
 // It does not generate one: a read cannot hand back keys that nothing will
-// write down (see ReadOrGenerateConfig). Nor does it refuse a document that
+// write down (see ReadConfigOrDefault). Nor does it refuse a document that
 // carries none, because a config legitimately has no identity between a logout
 // and the next login — mobile logout clears both keys in place — and this is
 // also the deserializer the iOS SDK copies a config through. Whoever goes on