[mle] introduce Connectivity and ConnectivityTlvValue types

[abtink]

This change separates the Connectivity TLV value format from its logical structure by introducing ConnectivityTlvValue (raw format) and Connectivity (parsed info). This replaces the ConnectivityTlv class and enables value format sharing between MLE and Network Diagnostics (without improper TLV inheritance).

It also updates ParentCandidate to use the new Connectivity class for better field encapsulation. It also updates ConnectivityTlvValue parsing to handle optional fields and enforce spec-defined minimums for these fields.

[gemini-code-assist]

Summary of Changes

Hello @abtink, 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 introduces a significant architectural improvement by decoupling the logical representation of connectivity data from its on-wire TLV format. This separation enhances code clarity, improves data encapsulation within key structures like ParentCandidate, and ensures a consistent approach to handling connectivity information across different modules such as MLE and Network Diagnostics. The update also brings more robust parsing capabilities for the Connectivity TLV, accommodating optional fields as per specification.

Highlights

• Separation of Concerns for Connectivity TLV: The Connectivity TLV data has been refactored into two distinct types: Connectivity to represent the logical structure of the connectivity information, and ConnectivityTlvValue to handle the raw, on-wire format of the TLV.
• Improved Data Encapsulation: The ParentCandidate structure now directly embeds the new Connectivity object, leading to better encapsulation of parent-related connectivity attributes and simplifying access to these fields.
• Shared TLV Format: This change enables the ConnectivityTlvValue format to be shared consistently between the MLE (Mesh Link Establishment) and Network Diagnostics modules, avoiding previous issues with improper TLV inheritance.
• Robust TLV Parsing: The parsing logic for ConnectivityTlvValue has been enhanced to correctly handle optional fields within the TLV and to enforce spec-defined minimum values for these fields when they are not explicitly present.
• Refactored Connectivity TLV Handling: The previous ConnectivityTlv class has been replaced by the new Connectivity and ConnectivityTlvValue types across the codebase, including message appending and reading functions.

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[gemini-code-assist]

Code Review

This pull request is a well-executed refactoring that separates the Connectivity TLV's raw format from its logical structure. The introduction of ConnectivityTlvValue for the raw format and Connectivity for the parsed information is a clean design choice that effectively replaces the old ConnectivityTlv class. This change successfully enables the sharing of the value format between MLE and Network Diagnostics, which reduces code duplication. Additionally, updating ParentCandidate to use the new Connectivity class improves encapsulation. The parsing logic for ConnectivityTlvValue has also been enhanced to handle optional fields, making it more robust. The changes are consistently applied across all relevant files, resulting in cleaner and more maintainable code. I have no specific comments as the implementation is solid.

[github-actions]

Merging #12300 into main

name	branch	text	data	bss	total
ot-cli-ftd	0d1cca3	482080	860	66548	549488
	72bdbc0	482080	860	66556	549496
	+/-	+0	+0	+8	+8
ot-ncp-ftd	0d1cca3	447700	764	61784	510248
	72bdbc0	447796	764	61792	510352
	+/-	+96	+0	+8	+104
ot-cli-mtd	0d1cca3	373488	764	50980	425232
	72bdbc0	373520	764	50980	425264
	+/-	+32	+0	+0	+32
ot-ncp-mtd	0d1cca3	354644	764	46248	401656
	72bdbc0	354676	764	46248	401688
	+/-	+32	+0	+0	+32
ot-cli-ftd-br	0d1cca3	589216	868	136460	726544
	72bdbc0	589200	868	136468	726536
	+/-	-16	+0	+8	-8
ot-rcp	0d1cca3	63264	568	20804	84636
	72bdbc0	63264	568	20804	84636
	+/-	+0	+0	+0	+0

Library files

name	branch	text	data	bss	total
libopenthread-ftd.a	0d1cca3	248964	95	40327	289386
	72bdbc0	248942	95	40335	289372
	+/-	-22	+0	+8	-14
libopenthread-cli-ftd.a	0d1cca3	61989	0	8091	70080
	72bdbc0	61989	0	8091	70080
	+/-	+0	+0	+0	+0
libopenthread-ncp-ftd.a	0d1cca3	33581	0	5948	39529
	72bdbc0	33581	0	5948	39529
	+/-	+0	+0	+0	+0
libopenthread-mtd.a	0d1cca3	166636	0	24791	191427
	72bdbc0	166718	0	24791	191509
	+/-	+82	+0	+0	+82
libopenthread-cli-mtd.a	0d1cca3	41426	0	8059	49485
	72bdbc0	41426	0	8059	49485
	+/-	+0	+0	+0	+0
libopenthread-ncp-mtd.a	0d1cca3	25805	0	5948	31753
	72bdbc0	25805	0	5948	31753
	+/-	+0	+0	+0	+0
libopenthread-ftd-br.a	0d1cca3	357718	100	110199	468017
	72bdbc0	357728	100	110207	468035
	+/-	+10	+0	+8	+18
libopenthread-cli-ftd-br.a	0d1cca3	80402	0	8131	88533
	72bdbc0	80402	0	8131	88533
	+/-	+0	+0	+0	+0
libopenthread-rcp.a	0d1cca3	9944	0	5060	15004
	72bdbc0	9944	0	5060	15004
	+/-	+0	+0	+0	+0
libopenthread-radio.a	0d1cca3	19547	0	238	19785
	72bdbc0	19547	0	238	19785
	+/-	+0	+0	+0	+0

[codecov]

Codecov Report

❌ Patch coverage is 93.26923% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.59%. Comparing base (0d1cca3) to head (501f7dd).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/thread/mle.cpp	86.20%	4 Missing ⚠️
src/core/thread/mle_tlvs.cpp	90.62%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12300      +/-   ##
==========================================
+ Coverage   76.09%   76.59%   +0.50%     
==========================================
  Files         680      677       -3     
  Lines       92485    93932    +1447     
==========================================
+ Hits        70378    71949    +1571     
+ Misses      22107    21983     -124     

Files with missing lines	Coverage Δ
src/core/thread/mle.hpp	93.47% <ø> (-1.53%)	⬇️
src/core/thread/mle_ftd.cpp	87.61% <100.00%> (+0.19%)	⬆️
src/core/thread/mle_tlvs.hpp	87.90% <ø> (-12.10%)	⬇️
src/core/thread/mle_types.cpp	100.00% <100.00%> (ø)
src/core/thread/mle_types.hpp	98.50% <100.00%> (-1.50%)	⬇️
src/core/thread/network_diagnostic.cpp	81.01% <100.00%> (+0.09%)	⬆️
src/core/thread/network_diagnostic_tlvs.hpp	100.00% <ø> (ø)
src/core/thread/mle_tlvs.cpp	95.65% <90.62%> (-4.35%)	⬇️
src/core/thread/mle.cpp	90.84% <86.20%> (+0.31%)	⬆️

... and 212 files with indirect coverage changes

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