Fix annotate client-side length check

[rianmcguire]

Description

Client-side validation of the annotation body length (< 1MiB) isn't working when the annotation body is read from STDIN - instead it always falls through to the server-side validation.

Before:

$ /bin/bash -e -c "ruby -e \"a = ('A'..'Z').to_a; (2 * 1024 * 1024).times { putc a.sample }\" | buildkite-agent annotate"
2024-04-12 09:23:30 INFO   Reading annotation body from STDIN command=annotate
fatal: failed to annotate build: POST https://agent.buildkite.com/v3/jobs/018ecf79-1b8c-48a7-9252-7a33cd980743/annotations: 400 Bad Request: The annotation body must be less than 1 MB

After:

$ /bin/bash -e -c "ruby -e \"a = ('A'..'Z').to_a; (2 * 1024 * 1024).times { putc a.sample }\" | buildkite-agent annotate"
2024-04-12 09:24:54 INFO   Reading annotation body from STDIN command=annotate
fatal: annotation body size (2097152B) exceeds maximum (1048576B)

Context

Support escalation

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[rianmcguire]

I considered adding a test for this, but it would require some additional indirection around stdin to make it testable. The added complication didn't seem worthwhile for the low risk here. Let me know if you disagree!

[DrJosh9000]

LGTM

[mdb]

Thanks!