[tailsamplingprocessor] Add gate for default OTTL error_mode ignore

[MukundaKatta]

Description

Adds an alpha feature gate for the tail sampling processor to switch the default ottl_condition.error_mode from propagate to ignore, matching the rollout requested in #48420. Explicit error_mode values are unchanged, and omitted values keep using propagate until processor.tailsamplingprocessor.defaultErrorModeIgnore is enabled.

Also updates the config docs/schema and covers both default paths in a unit test.

Link to tracking issue

Fixes #48420

Testing

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

[github-actions]

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