refactor: replace func-typed injection seams in pkg/pchain with interfaces

[owenwahlgren]

Summary

The nine issue*Tx helpers in pkg/pchain/pchain.go took the avalanchego wallet call as a function-typed parameter so unit tests could stub it. With multi-parameter builders the inline function types grew longer than the helper bodies — and the pattern was recently copied in #28 into a 10-parameter version that drew, let's say, feedback from upstream.

This replaces each seam with a single-method issuer interface mirroring the corresponding wallet/chain/p wallet method, so w.PWallet() satisfies them directly:

// Before
func issueSendTx(
    issueBaseTx func(outputs []*avax.TransferableOutput, options ...common.Option) (*txs.Tx, error),
    avaxAssetID ids.ID, to ids.ShortID, amountNAVAX uint64, options ...common.Option,
) (ids.ID, error)

// After
type baseTxIssuer interface {
    IssueBaseTx(outputs []*avax.TransferableOutput, options ...common.Option) (*txs.Tx, error)
}

func issueSendTx(issuer baseTxIssuer, avaxAssetID ids.ID, to ids.ShortID, amountNAVAX uint64, options ...common.Option) (ids.ID, error)

• No function-typed parameters remain in pkg/pchain (grep-verified)
• No behavior change: exported API (Send, AddValidator, CreateSubnet, …), error messages, and %w wrapping identical
• Tests converted from inline closures to small stub structs with capture fields; all 11 tests keep identical assertions (arg mapping for amounts, owners, durations, shares, signer, option propagation, error wrapping)
• This also establishes the pattern feat: support ACP-236 auto-renewed staking commands #28 should rebase onto for AddAutoRenewedValidatorTx

Note: includes the same whitespace-only gofmt fixes as #29 (five files unformatted on main) — whichever merges second rebases trivially.

Validation

• gofmt -l . clean, go vet ./... clean, staticcheck v0.6.1 clean (plain + e2e tags)
• go test -race ./pkg/... ./cmd/... all pass; -tags=integration compile check passes
• go test -tags=clie2e ./e2e/... -run "Help|Params|MissingArgs" passes