Consolidate TransactionPipeline's internal retry loop onto utils/retry.ts

[bbjiggy]

Description

Follow-up from the #79 spike (see docs/spikes/issue-79-retry-session-multisig.md).

TransactionPipeline (src/tx-pipeline/pipeline.ts) implements its own local withRetry helper
(exponential backoff, per-stage error wrapping). src/utils/retry.ts exports a generic retry()
helper with a very similar exponential-backoff shape, but a different signature (fixed
attempts/delay vs. a policy object, no per-attempt callback, no stage-aware error wrapping). The
two aren't unified, so there are two backoff implementations to maintain.

Proposal

Refactor withRetry in tx-pipeline/pipeline.ts to build on top of retry() from
src/utils/retry.ts (e.g. by extending retry() to accept a delay-strategy function and an
optional per-attempt callback, or by having withRetry call retry() internally), removing the
duplicated backoff loop while preserving TransactionPipeline's existing stage-aware
TrustFlowError wrapping and RetryPolicy (min/max delay) semantics.

This is a refactor of tested, shipped code (tests/tx-pipeline.test.ts, tests/retry.test.ts),
not a new feature — scope it as its own reviewable change rather than bundling it with other work.

Acceptance Criteria

• Single shared backoff implementation used by both utils/retry.ts consumers and TransactionPipeline
• TransactionPipeline's existing public behavior (retry counts, delay caps, error codes) unchanged
• Existing tx-pipeline and retry test suites continue to pass without behavioral changes

[Securify001]

@Securify001 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi. I am a web3 fullstack developer. Kindly assign this issue to me

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