fix: add nil guards for Solana tx metadata and safe signature parsing (backport #4550 to v37)

[ws4charlie]

Summary

• Backport of fix: add nil guards for Solana tx metadata and safe signature parsing #4550 and fix: reject Solana transactions with nil metadata instead of treating as successful #4557 to release/zetaclient/v37 (ZetaChain mainnet)
• Add Meta != nil guard before accessing txResult.Meta.Err in three locations to prevent potential nil pointer dereference
• Replace MustSignatureFromBase58 (panics on invalid input) with SignatureFromBase58 (returns error) in inbound_tracker.go
• Explicitly reject Solana transactions with nil Meta in inbound/outbound observers instead of silently treating them as successful
• In the outbound tracker reporter, continue monitoring (rather than giving up) when metadata is not yet available

Addresses review feedback from #4552 (comment): the previous Meta != nil && Meta.Err != nil guard allowed transactions with unknown execution status (nil metadata) to proceed through processing and voting.

Before/After

Scenario	Before	After
txResult.Meta == nil in observer	Silently treated as successful tx	Explicitly rejected with error log
tx.Meta == nil in tracker reporter	Silently treated as successful tx	Continues polling until metadata appears (or timeout)

Test plan

• CI passes

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 5.55556% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...taclient/chains/solana/observer/inbound_tracker.go	0.00%	8 Missing ⚠️
zetaclient/chains/solana/observer/outbound.go	0.00%	2 Missing and 1 partial ⚠️
.../chains/solana/signer/outbound_tracker_reporter.go	0.00%	3 Missing ⚠️
zetaclient/chains/solana/observer/inbound.go	0.00%	1 Missing and 1 partial ⚠️
...etaclient/chains/solana/observer/inbound_parser.go	50.00%	0 Missing and 1 partial ⚠️

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