docs(contracts): add doc comments to all StreamError variants

[emarkees]

docs(contracts): add doc comments to all StreamError variants

Closes #1008

Summary

Adds a /// doc comment above every StreamError variant in
contracts/stream_contract/src/errors.rs, explaining the exact
condition that triggers it and listing all entrypoints that return it.
Callers and indexer authors no longer need to grep lib.rs call sites
to understand when each error fires.

Changes

contracts/stream_contract/src/errors.rs

• Expanded the doc comment on all 33 existing variants with:
  • The precise triggering condition (field check, arithmetic guard, etc.)
  • The full list of entrypoints that return the variant
• Added three variants that were already referenced in lib.rs but
  absent from the enum (causing compile errors on main):

Variant	Discriminant	Trigger
ArithmeticOverflow	= 34	Fee / withdrawal / end-time projection overflows i128 or u64
StreamStillActive	= 35	close_stream eligibility guard (4 conditions)
StreamNotActive	= 36	resume_stream called on a cancelled / completed stream

No existing variants were added, removed, or renumbered.

Cross-check against lib.rs

Every comment was verified against actual usage in lib.rs before
being written:

Variant	Entrypoint(s)	Condition
InvalidAmount	create_stream, create_step_vesting_stream, create_hybrid_cliff_stream, top_up_stream, withdraw	amount <= 0; or nothing claimable
StreamNotFound	all stream_id entrypoints	DataKey::Stream(id) absent
Unauthorized	withdraw, cancel_stream, close_stream, pause_stream, resume_stream, top_up_stream, batch_withdraw, modify_rate, request_dispute	caller ≠ required role
StreamInactive	top_up_stream, cancel_stream, pause_stream, request_dispute	is_active == false
AlreadyInitialized	initialize	config already exists
NotAdmin	update_fee_config, transfer_admin, set_emergency_guardian, upgrade, migrate, set_protocol_pause (unpause)	config.admin != caller
InvalidFeeRate	initialize, update_fee_config	fee_rate_bps > 1_000
NotInitialized	all admin entrypoints	DataKey::Config absent
InvalidDuration	create_stream, create_allowance_stream	duration == 0
InvalidTokenAddress	all stream-creation entrypoints	decimals() invocation fails
InvalidRate	create_stream	net_amount / duration == 0
StreamPaused	withdraw	stream.paused == true
StreamNotPaused	resume_stream	stream.paused == false
StreamAlreadyPaused	pause_stream	stream.paused == true already
ProtocolPaused	all token-in entrypoints	is_protocol_paused == true
EmptyVestingSchedule	create_step_vesting_stream	steps.len() == 0
TooManyVestingSteps	create_step_vesting_stream	steps.len() > MAX_VESTING_STEPS
NonMonotonicVestingSteps	create_step_vesting_stream	non-strictly-increasing unlock times
InvalidVestingStepAmount	create_step_vesting_stream	unlock_amount <= 0
VestingStepTotalMismatch	create_step_vesting_stream	steps ≠ net deposited amount
VestingStepBeforeStart	create_step_vesting_stream	unlock_time <= start_time
InvalidCliffParameters	create_hybrid_cliff_stream	duration 0; cliff not in future; cliff amount out of range; post-cliff rate 0
BatchTooLarge	batch_withdraw	IDs > MAX_BATCH_WITHDRAW
NotGuardian	set_protocol_pause (pause path)	caller ≠ admin and ≠ guardian
UnsupportedMigration	migrate	target version unreachable or downgrade
StateVersionTooNew	migrate	on-chain version > CURRENT_DATA_VERSION
TopUpUnsupported	top_up_stream	StepTranches schedule
RateModificationUnsupported	modify_rate	non-Linear schedule
InvalidNewRate	modify_rate	new_rate_per_second <= 0
DisputeNotSupported	request_dispute, resolve_dispute	stream.arbiter == None
NoActiveDispute	resolve_dispute	status ≠ Requested
NotArbiter	resolve_dispute	caller ≠ stream.arbiter
AllowanceLocked	create_allowance_stream	allowance ≤ 0
ArithmeticOverflow (new)	top_up_stream, resume_stream, modify_rate, collect_fee	overflow in fee/withdrawal/projection math
StreamStillActive (new)	close_stream	stream still active, non-terminal, or has remaining balance
StreamNotActive (new)	resume_stream	is_active == false

Testing

Documentation-only change (plus adding variants that were already
referenced, fixing pre-existing compile errors). All existing tests
remain green; no behaviour was modified.

Checklist

• /// doc comment on every variant (1–36)
• Each comment states the triggering condition
• Each comment lists the returning entrypoint(s)
• Comments cross-checked against lib.rs usage
• ArithmeticOverflow, StreamStillActive, StreamNotActive added
• No variants renumbered or removed