fix(supervisor): bound pending exec stdin and cancel stalled writers

[shiju-nv]

Summary

Bound pending stdin to 4 MiB per ordinary SSH exec channel, including the active write, and release stalled input writers on channel close or disconnect. Overflow reports exit code 74 and requests termination of the affected exec while keeping the SSH callback responsive to other channel traffic.

Related Issue

No issue required: this corrects an unbounded queue and detached input writer in the existing exec path.

Changes

• Replace the unbounded queue and detached OS thread with a byte buffer and cancellable async writer. Coalesce small packets and reject excess input before copying it.
• Preserve EOF draining and reuse capacity as input is written. Slow consumers that exceed the pending-input budget receive a failed exec; the supervisor logs the reason.
• Keep overflow handling independent of output credit. Do not enqueue diagnostic stderr that could delay another channel's completion.
• Replace tests that duplicated the old writer loop with coverage through the real SSH handler, and document the overload behavior.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

Label test:e2e applied for 2595b40. Open the existing run and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway, sandbox, and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.

[johntmyers]

gator-agent

PR Review Status

The initial code-only review found no blocking findings. The supervisor-process change is project-valid as a localized resource-bound and cancellation fix, its user-visible overload behavior is documented, and the required E2E workflow is queued for this head.

Blocking findings:

• No blocking findings remain

Carried findings:

• None

Non-blocking suggestions:

• None

Gator metadata

• Validation: Localized supervisor correctness fix with handler-level regression coverage
• Docs: Fern documentation updated in docs/how-it-works/sandboxes/overview.mdx
• Checks: Current-head baseline checks completed; required E2E is queued
• E2E: test:e2e applied and Branch E2E Checks run 36630140527 rerun queued
• Head SHA: 2595b40f6f060437ccb5b29c1145b499e9d80bcd
• Base SHA: 33a8eac196a21aff3125dea46462a4591bcb5d0b
• Merge base SHA: 33a8eac196a21aff3125dea46462a4591bcb5d0b
• Patch ID: a296c71d0634a50b6252e3a70c1dd620c426e2c6
• Gator payload: 9
• Review mode: initial
• Previous reviewed SHA: none
• Review budget exhausted: no
• Maintainer decision required: no
• Next state: gator:watch-pipeline