fix: lower log level for missing base ref detection (EME-369)

Changes the logging level from warn to info when failing to detect base
branch reference in build uploads. This reduces noise in build logs for
common scenarios including detached HEAD states in PRs and main/master
branch builds where base ref detection failures are expected.

Also removes --log-level=error from all build upload tests and updates
test expectations to include the experimental warning that is now visible.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: runningcode

src/commands/build/upload.rs

@@ -192,7 +192,7 @@ pub fn execute(matches: &ArgMatches) -> Result<()> {
                             Some(base_ref_name)
                         }
                         Err(e) => {
-                            warn!("Could not detect base branch reference: {}", e);
+                            info!("Could not detect base branch reference: {}", e);
                             None
                         }
                     })

tests/integration/_cases/build/build-upload-apk-all-uploaded.trycmd

@@ -1,6 +1,7 @@
 ```
-$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk --log-level=error
+$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk
 ? success
+  WARN    [..] EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
 Successfully uploaded 1 file to Sentry
   - tests/integration/_fixtures/build/apk.apk (http[..]/wat-org/preprod/wat-project/42)
 

tests/integration/_cases/build/build-upload-apk-no-token.trycmd

@@ -1,6 +1,7 @@
 ```
-$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk --log-level=error
+$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk
 ? failed
+  WARN    [..] EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
 error: Auth token is required for this request. Please run `sentry-cli login` and try again!
 
 Add --log-level=[info|debug] or export SENTRY_LOG_LEVEL=[info|debug] to see more output.

tests/integration/_cases/build/build-upload-apk.trycmd

@@ -1,6 +1,7 @@
 ```
-$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk --head-sha test_head_sha --log-level=error
+$ sentry-cli build upload tests/integration/_fixtures/build/apk.apk --head-sha test_head_sha
 ? success
+  WARN    [..] EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
 Successfully uploaded 1 file to Sentry
   - tests/integration/_fixtures/build/apk.apk (http[..]/wat-org/preprod/wat-project/42)
 

tests/integration/_cases/build/build-upload-ipa.trycmd

@@ -1,6 +1,7 @@
 ```
-$ sentry-cli build upload tests/integration/_fixtures/build/ipa.ipa --head-sha test_head_sha --log-level=error
+$ sentry-cli build upload tests/integration/_fixtures/build/ipa.ipa --head-sha test_head_sha
 ? success
+  WARN    [..] EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
 Successfully uploaded 1 file to Sentry
   - tests/integration/_fixtures/build/ipa.ipa (http[..]/wat-org/preprod/wat-project/some-text-id)