[network-diag] introduce RouterNeighborTlv

[abtink]

This commit updates RouterNeighborTlv to follow the SimpleTlvInfo pattern, separating the TLV value structure from its type/length header.

Specifically, it defines RouterNeighborTlvValue to hold the data fields for a router neighbor's diagnostic information, while RouterNeighborTlv is redefined as a SimpleTlvInfo using the value structure and the kRouterNeighbor type.

This allows the use of generic Tlv::Append<RouterNeighborTlv>() and tlvInfo.Read<RouterNeighborTlv>() methods, which are generally safer and allow value type reuse.

The NetworkDiagnostic and MeshDiag module are updated to utilize these new methods.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the RouterNeighborTlv implementation to adopt the standard SimpleTlvInfo pattern used across the codebase. By decoupling the TLV value structure from the type/length header, the changes improve code safety and maintainability while allowing for more consistent use of generic TLV handling utilities throughout the network diagnostic and mesh diagnostic modules.

Highlights

• Refactoring RouterNeighborTlv: Updated RouterNeighborTlv to follow the SimpleTlvInfo pattern by separating the TLV value structure from its header.
• API Improvements: Introduced RouterNeighborTlvValue to hold data fields, enabling the use of generic Tlv::Append and tlvInfo.Read methods for improved safety and reusability.
• Module Updates: Updated NetworkDiagnostic and MeshDiag modules to utilize the new TLV structure and reading methods.

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[github-actions]

Merging #12827 into main

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ot-ncp-ftd	dab83c9	448676	764	61776	511216
	67113d9	448676	764	61776	511216
	+/-	+0	+0	+0	+0
ot-cli-mtd	dab83c9	374928	764	50972	426664
	67113d9	374928	764	50972	426664
	+/-	+0	+0	+0	+0
ot-ncp-mtd	dab83c9	356076	764	46240	403080
	67113d9	356076	764	46240	403080
	+/-	+0	+0	+0	+0
ot-cli-ftd-br	dab83c9	599128	864	136612	736604
	67113d9	599112	864	136612	736588
	+/-	-16	+0	+0	-16
ot-rcp	dab83c9	63328	568	20804	84700
	67113d9	63328	568	20804	84700
	+/-	+0	+0	+0	+0

Library files

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libopenthread-cli-ftd.a	dab83c9	61750	0	8091	69841
	67113d9	61750	0	8091	69841
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libopenthread-ncp-ftd.a	dab83c9	33583	0	5948	39531
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libopenthread-mtd.a	dab83c9	169022	0	24783	193805
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libopenthread-cli-mtd.a	dab83c9	41459	0	8059	49518
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libopenthread-ncp-mtd.a	dab83c9	25807	0	5948	31755
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libopenthread-ftd-br.a	dab83c9	367774	99	110351	478224
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libopenthread-cli-ftd-br.a	dab83c9	80957	0	8131	89088
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libopenthread-rcp.a	dab83c9	9946	0	5060	15006
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libopenthread-radio.a	dab83c9	19725	0	238	19963
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[gemini-code-assist]

Code Review

This pull request refactors the RouterNeighborTlv implementation by introducing RouterNeighborTlvValue and utilizing the SimpleTlvInfo template. These changes simplify TLV appending and reading operations across the network diagnostic and mesh diagnostic modules. Feedback was provided regarding a potential regression in forward compatibility: the new implementation uses a strict length check when reading the TLV, whereas the previous code allowed for a minimum length, which is safer if the protocol is extended in the future.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.57%. Comparing base (dab83c9) to head (7934981).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/thread/network_diagnostic.cpp	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12827      +/-   ##
==========================================
- Coverage   75.56%   74.57%   -1.00%     
==========================================
  Files         694      696       +2     
  Lines       95495    95483      -12     
==========================================
- Hits        72165    71205     -960     
- Misses      23330    24278     +948     

Files with missing lines	Coverage Δ
src/core/thread/network_diagnostic_tlvs.cpp	81.75% <100.00%> (-0.27%)	⬇️
src/core/thread/network_diagnostic_tlvs.hpp	97.67% <ø> (ø)
src/core/utils/mesh_diag.cpp	85.22% <100.00%> (-0.06%)	⬇️
src/core/utils/mesh_diag.hpp	50.00% <ø> (ø)
src/core/thread/network_diagnostic.cpp	67.87% <50.00%> (-10.77%)	⬇️

... and 217 files with indirect coverage changes

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