--config override: decode errors name the file "." (filepath.Base self-assignment)

[fchimpan]

Summary

--config <file>.yaml loads the override file but computes the stored file name from the wrong variable, so the name becomes "." and decode errors read decode .: instead of naming the user's file.

Reproduction

printf 'version: v2\nbadkey: {{{' > custom.yaml
buf build --config custom.yaml
# Failure: decode .: invalid character 'v' looking for beginning of value
#          ^^^^^^^^ should be "decode custom.yaml:"

(Verified against a local build — the output above is real.)

Root cause

private/bufpkg/bufconfig/buf_yaml_file.go:191-197 (GetBufYAMLFileForOverride):

var fileName string
...
case ".json", ".yaml", ".yml":
	data, err = os.ReadFile(override)
	...
	fileName = filepath.Base(fileName)   // self-assignment; fileName is still ""

Should be fileName = filepath.Base(override). filepath.Base("") returns ".", which then becomes the ObjectData name used in error messages.

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Found via a full mutest (mutation-testing) run over this repo; verified manually.