[processor/tailsampling] Gate OTTL condition error mode default

[MukundaKatta]

Description

Adds an alpha feature gate for changing the default error_mode of ottl_condition policies from propagate to ignore when the field is omitted. Explicitly configured error_mode values continue to take precedence.

Fixes #48420

Testing

• go test . from processor/tailsamplingprocessor
• git diff --check

[github-actions]

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