apollo_propeller: add inbound_channel_capacity config field

[sirandreww-starkware]

Note

Low Risk
Config-only change (new field + default) with no behavioral impact until consumers start using the new parameter.

Overview
Adds a new Config::inbound_channel_capacity setting to parameterize the bounded handler→engine inbound channel size, with documentation describing its back-pressure behavior and a default of 256.

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[reviewable-StarkWare]

This change is 

[sirandreww-starkware]

• apollo_propeller: route inbound units through bounded channel #13350 : 2 dependent PRs (#13582 , #14533 )
• apollo_propeller: buffer unsent units from partial batches #13349
• apollo_propeller: wire bounded unit channels between handler and engine #13348
• apollo_propeller: add RegisterHandler command and inbound unit SelectAll #13347
• apollo_propeller: add inbound_channel_capacity config field #13346 👈 (View in Graphite)
• main

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[guy-starkware]

@guy-starkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ShahakShama).

[cursor]

PR Summary

Low Risk
Config-only addition with a conservative default; no behavior change until callers wire the field into channel creation.

Overview
Adds inbound_channel_capacity to propeller Config, documenting it as the bounded channel size between each handler and the engine for inbound units.

When the channel is full, handlers stop reading from the network so yamux flow control can slow the remote peer. Default sets the value to 16.

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[ShahakShama]

@ShahakShama reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on sirandreww-starkware).