feat(lambda): read cloud.account.id from symlink in Lambda detector

[RaphaelManke]

Summary

• Reads cloud.account.id from the symlink at /tmp/.otel-account-id created by the OTel Lambda Extension
• Uses File.readlink() to read the symlink target
• Rescues Errno::ENOENT and Errno::EINVAL to silently skip if the symlink doesn't exist

Depends on

• feat(lambda): write cloud.account.id symlink from extension opentelemetry-lambda#2127 (extension creates the symlink)

Changes

• lambda.rb — Added readlink logic with ACCOUNT_ID_SYMLINK_PATH constant
• Tests: happy path, leading zeros, missing symlink, not-a-symlink

Test plan

• Unit test: symlink exists → cloud.account.id set correctly
• Unit test: leading zeros preserved as string
• Unit test: missing symlink → attribute absent, no error
• Unit test: non-symlink file → attribute absent, no error