fix(excon): enable SimpleCov coverage merging across appraisal runs

[nicknikolakakis]

Summary

• Set a unique SimpleCov.command_name per appraisal gemfile so coverage data from each semantic convention mode (stable, old, dup) is preserved and merged rather than overwritten
• This brings combined test coverage from ~58% to 99.19%, meeting the required 85% threshold

Root cause

The excon instrumentation tests 3 semantic convention modes via separate appraisal gemfiles. Each test run loads code for all 3 modes but only exercises one (the others are skipped based on BUNDLE_GEMFILE). Without a unique command_name, each run overwrites the previous one's entry in SimpleCov's .resultset.json, so coverage never reflects the full test suite.

Fix

One line in test_helper.rb derives a unique command_name from BUNDLE_GEMFILE:

SimpleCov.command_name(ENV.fetch('BUNDLE_GEMFILE', 'default').split('/').last.sub('.gemfile', ''))

This produces names like excon_latest_stable, excon_1.3.1_old, etc., allowing SimpleCov to merge results across all appraisal runs.

Coverage

Metric	Before	After
Per-run coverage	~58%	~58% (unchanged per run)
Merged coverage	~58% (overwrites)	99.19% (proper merge)

The 3 remaining uncovered lines (0.81%) are the rescue StandardError => e safety nets in each middleware's handle_response.

Fixes #2155

Test plan

• Verified locally: ran all 6 appraisals and confirmed SimpleCov merges coverage to 99.19%
• Verified SimpleCov.minimum_coverage(85) passes after merge
• RuboCop passes on the changed file