redis_streams: add interpolation support for entry ID

[twmb]

Allow users to specify a custom entry ID for the redis_streams output using Bloblang interpolation, instead of always auto-generating with "*". This enables replaying messages in upstream order.

Closes #1242

[claude]

Claude finished @twmb's task in 1m 51s —— View job

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Commits
LGTM

Review
Clean addition of an interpolated id field to the redis_streams output, preserving backwards compatibility with the "*" default. Config spec, field constant, struct field, parsing, and both WriteBatch code paths (single and batched) are all consistent with project patterns.

1. t.Context() used inside t.Cleanup() in both test subtests (lines 142 and 174) — should be context.Background() since t.Context() is already canceled during cleanup.

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[claude]

Claude finished @twmb's task in 1m 5s —— View job

---

• Fetch PR data and read agent guidelines
• Validate commit policy
• Review code changes
• Post summary

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Commits
LGTM

Review
Clean addition of an interpolated id field to the redis_streams output, preserving backwards compatibility with the "*" default. The previous review's t.Context() concern in t.Cleanup() has been addressed — both cleanup calls now correctly use context.Background().

LGTM

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